I'm seeing quite a few

  ModSecurity: Multipart parsing error: Multipart: writing to "/tmp/20130813-094051-UgnjA

on uploads of multiple files.

  SecDataDir /tmp
  SecTmpDir /tmp
  SecUploadDir /tmp/
  SecUploadFileMode 644

are all in place and disk space is not an issue.

These errors are not reproducible reliably, meaning I have to try the upload several times before it happens. I do see some temporary files being created but not to the point where

  SecUploadFileLimit

would hit. It is not set explicitly, so defaults to 100 according to https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#wiki-SecUploadFileLimit. Between the attempts to reproduce the error there are no "left-overs" in /tmp/ so this is not due to /tmp/ filling up.

This is happening on mod_security 2.5.12 with OWASP CRS 2.7.3. I'm aware this stuff is rather old but I have to be absolutely, 100% sure an update fixes this issue before updating becomes an option.

Any suggestions on how to debug this ?

Cheers,
  Thomas