Same thing happens here since migrating from 0.49 to 0.50. We have the vast majority of our subscriptions that are no longer being sent to Slack. We no longer capture them in their respective Slack channel or in the #metabase_files where everything is supposed to be captured. We appear to have intermittent subscription being sent through, but for 95% of them, they don't get to Slack. Email subscriptions work perfectly fine. The error message appears to be around the fact that the connection was dropped or timed out after a bit of time. This is the full error message I'm seeing.
ERROR metabase.pulse Error sending notification!,java.net.SocketException: Connection reset, at java.base/java.net.SocketInputStream.read(Unknown Source), at java.base/java.net.SocketInputStream.read(Unknown Source), at java.base/sun.security.ssl.SSLSocketInputRecord.read(Unknown Source), at java.base/sun.security.ssl.SSLSocketInputRecord.readHeader(Unknown Source), at java.base/sun.security.ssl.SSLSocketInputRecord.decode(Unknown Source), at java.base/sun.security.ssl.SSLTransport.decode(Unknown Source), at java.base/sun.security.ssl.SSLSocketImpl.decode(Unknown Source), at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(Unknown Source), at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(Unknown Source), at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(Unknown Source), at org.apache.http.conn.ssl.SSLConnectionSocketFactory.createLayeredSocket(SSLConnectionSocketFactory.java:436), at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:384), at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:142), at org.apache.http.impl.conn.BasicHttpClientConnectionManager.connect(BasicHttpClientConnectionManager.java:313), at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:393), at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236), at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:186), at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:89), at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110), at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185), at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83), at clj_http.core$request.invokeStatic(core.clj:621), at clj_http.core$request.invoke(core.clj:546), at clj_http.core$request.invokeStatic(core.clj:547), at clj_http.core$request.invoke(core.clj:546), at clojure.lang.Var.invoke(Var.java:384), at clj_http.client$wrap_request_timing$fn__41317.invoke(client.clj:1083), at clj_http.headers$wrap_header_map$fn__40284.invoke(headers.clj:152), at clj_http.client$wrap_query_params$fn__41207.invoke(client.clj:817), at clj_http.client$wrap_basic_auth$fn__41213.invoke(client.clj:840), at clj_http.client$wrap_oauth$fn__41218.invoke(client.clj:857), at clj_http.client$wrap_user_info$fn__41227.invoke(client.clj:877), at clj_http.client$wrap_url$fn__41299.invoke(client.clj:1035), at clj_http.client$wrap_decompression$fn__40994.invoke(client.clj:416), at clj_http.client$wrap_input_coercion$fn__41119.invoke(client.clj:630), at clj_http.client$wrap_additional_header_parsing$fn__41144.invoke(client.clj:685), at clj_http.client$wrap_output_coercion$fn__41106.invoke(client.clj:574), at clj_http.client$wrap_exceptions$fn__40947.invoke(client.clj:250), at clj_http.client$wrap_accept$fn__41159.invoke(client.clj:728), at clj_http.client$wrap_accept_encoding$fn__41166.invoke(client.clj:750), at clj_http.client$wrap_content_type$fn__41153.invoke(client.clj:711), at clj_http.client$wrap_form_params$fn__41264.invoke(client.clj:959), at clj_http.client$wrap_nested_params$fn__41285.invoke(client.clj:993), at clj_http.client$wrap_flatten_nested_params$fn__41294.invoke(client.clj:1017), at clj_http.client$wrap_method$fn__41232.invoke(client.clj:893), at clj_http.cookies$wrap_cookies$fn__40196.invoke(cookies.clj:128), at clj_http.links$wrap_links$fn__40624.invoke(links.clj:63), at clj_http.client$wrap_unknown_host$fn__41302.invoke(client.clj:1046), at clj_http.client$request_STAR_.invokeStatic(client.clj:1174), at clj_http.client$request_STAR_.invoke(client.clj:1167), at clj_http.client$post.invokeStatic(client.clj:1192), at clj_http.client$post.doInvoke(client.clj:1188), at clojure.lang.RestFn.invoke(RestFn.java:423), at metabase.integrations.slack$upload_file_to_url_BANG_.invokeStatic(slack.clj:375), at metabase.integrations.slack$upload_file_to_url_BANG_.invoke(slack.clj:374), at metabase.integrations.slack$upload_file_BANG_.invokeStatic(slack.clj:392), at metabase.integrations.slack$upload_file_BANG_.invoke(slack.clj:380), at metabase.pulse$create_and_upload_slack_attachments_BANG_$fn__99705.invoke(pulse.clj:324), at clojure.core.protocols$fn__8249.invokeStatic(protocols.clj:168), at clojure.core.protocols$fn__8249.invoke(protocols.clj:124), at clojure.core.protocols$fn__8204$G__8199__8213.invoke(protocols.clj:19), at clojure.core.protocols$seq_reduce.invokeStatic(protocols.clj:31), at clojure.core.protocols$fn__8236.invokeStatic(protocols.clj:75), at clojure.core.protocols$fn__8236.invoke(protocols.clj:75), at clojure.core.protocols$fn__8178$G__8173__8191.invoke(protocols.clj:13), at clojure.core$reduce.invokeStatic(core.clj:6887), at clojure.core$reduce.invoke(core.clj:6869), at metabase.pulse$create_and_upload_slack_attachments_BANG_.invokeStatic(pulse.clj:317), at metabase.pulse$create_and_upload_slack_attachments_BANG_.invoke(pulse.clj:308), at metabase.pulse$create_and_upload_slack_attachments_BANG_.invokeStatic(pulse.clj:314), at metabase.pulse$create_and_upload_slack_attachments_BANG_.invoke(pulse.clj:308), at metabase.pulse$fn__99865.invokeStatic(pulse.clj:513), at metabase.pulse$fn__99865.invoke(pulse.clj:511), at clojure.lang.MultiFn.invoke(MultiFn.java:229), at clojure.lang.AFn.applyToHelper(AFn.java:154), at clojure.lang.AFn.applyTo(AFn.java:144), at clojure.core$apply.invokeStatic(core.clj:667), at clojure.core$apply.invoke(core.clj:662), at metabase.util.retry$decorate$fn$reify__73939.call(retry.clj:77), at io.github.resilience4j.retry.Retry.lambda$decorateCallable$5(Retry.java:306), at metabase.util.retry$decorate$fn__73938.doInvoke(retry.clj:78), at clojure.lang.RestFn.applyTo(RestFn.java:137), at clojure.core$apply.invokeStatic(core.clj:667), at clojure.core$apply.invoke(core.clj:662), at metabase.pulse$send_notification_retrying_BANG_.invokeStatic(pulse.clj:533), at metabase.pulse$send_notification_retrying_BANG_.doInvoke(pulse.clj:530), at clojure.lang.RestFn.invoke(RestFn.java:408), at metabase.pulse$send_notifications_BANG_$fn__99883.invoke(pulse.clj:540), at metabase.pulse$send_notifications_BANG_.invokeStatic(pulse.clj:539), at metabase.pulse$send_notifications_BANG_.invoke(pulse.clj:535), at metabase.pulse$send_pulse_BANG_.invokeStatic(pulse.clj:564), at metabase.pulse$send_pulse_BANG_.doInvoke(pulse.clj:544), at clojure.lang.RestFn.invoke(RestFn.java:439), at metabase.task.send_pulses$send_pulse_BANG_$fn__104895.invoke(send_pulses.clj:57), at metabase.models.task_history$do_with_task_history.invokeStatic(task_history.clj:115), at metabase.models.task_history$do_with_task_history.invoke(task_history.clj:104), at metabase.task.send_pulses$send_pulse_BANG_.invokeStatic(send_pulses.clj:52), at metabase.task.send_pulses$send_pulse_BANG_.invoke(send_pulses.clj:49), at metabase.task.send_pulses$send_pulse_BANG__STAR_.invokeStatic(send_pulses.clj:125), at metabase.task.send_pulses$send_pulse_BANG__STAR_.invoke(send_pulses.clj:116), at metabase.task.send_pulses.SendPulse.execute(send_pulses.clj:147), at org.quartz.core.JobRunShell.run(JobRunShell.java:202), at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:573)