This is actually your web server configuration issue
If you take a closer look at the output of your web server log file, you will see that same error on three different scripts
[Fri Sep 20 08:28:48.941933 2024] [core:error] [pid 2239539:tid 140140661327616] (70007)The timeout specified has expired: [client 136.226.199.77:33215] AH00574: ap_content_length_filter: apr_bucket_read() failed, referer: .../wp-admin/edit.php
[Fri Sep 20 16:08:25.439776 2024] [core:error] [pid 1692686:tid 140669048571648] (70007)The timeout specified has expired: [client 147.161.131.107:37042] AH00574: ap_content_length_filter: apr_bucket_read() failed, referer: .../wp-admin/plugins.php
[Fri Sep 20 16:09:03.407928 2024] [core:error] [pid 1692686:tid 140669216425728] (70007)The timeout specified has expired: [client 147.161.131.107:37042] AH00574: ap_content_length_filter: apr_bucket_read() failed, referer: .../wp-admin/admin.php?page=acymailing_dashboard
The issue every time is a timeout that happens on a web server level
AH00574: ap_content_length_filter: apr_bucket_read() failed
Searching the internet for this problem suggests that the problem is with the timeout set on the web server level
Please reach out to your hosting support and report this problem to them because it is a widespread problem that happens in other places on WordPress and not just AcyMailing