Facing issues in uploading attachments to roundcube ? Last day, was not able to attach a PDF through webmail.

Apache error logs showed the following :

[Date [:error] [pid 12345:tid 123456789] [client xx.xx.xx.xx] [client xx.xx.xx.xx] ModSecurity: Access denied with code 44 (phase 2). Match of "eq 0" against "MULTIPART_UNMATCHED_BOUNDARY" required.

Even though mod_security isn’t loaded, faced this issue.

Error was a bug in Plesk. To fix, go to Plesk – Tools & Settings > Web Application Firewall (ModSecurity) and enable it, then disable it.