Cannot send any emails with attachments

[hnefatafl]

I am using v1.6.0 on Ubuntu Linux 22.04.3.

Roundcube has been set up and working well for years, but recently no user is able to send an email with an attachment, regardless of the size. We are met with the warning "Connection Error. (Failed to reach the server)!"  Sending and receiving emails without attachments works as intended.

The temp directory is fully writable (/var/www/html/roundcube/temp/). The log files don't show anything after Sep though.

I'm at a loss.  Is there something else I should be looking for?

[JohnDoh]

It sounds a bit like https://github.com/roundcube/roundcubemail/issues/7494 are you running any request filtering on your webserver which could be blocking the request before it gets to Roundcube?

[hnefatafl]

Thanks for that, but no; I don't think mine's related to that one. I'm not running any sort of request filtering, and I can receive emails from all sources I've tested (Outlook from two locations at work, GMail, another private email server). I can receive emails with attachments without issues, and every client can send emails to any recipient, unless it has an attachment. 

I though something might be wrong with the temp folder, but it doesn't appear to be.