This sounds like an issue with mySQL setup. I assume that you are saving files to the DATABASE.

Is it all large files that fail? If so, you could look at the "max_allowed_packet" parameter in the my.cnf file in your installation.

I just experienced a similar issue on my mantis installation. I had no "max_allowed_packet" configured on my.cnf. Setting it to 16M, according to instructions in the following page, solved the problem:

http://dev.mysql.com/doc/mysql/en/packet-too-large.html

Are the docs and admin/check.php clear on this?

I had the same problem. My "max_allowed_packet" was configured to 1 MB (/etc/my.cnf). I changed it to the same size as the maximum allowed upload size from PHP. Now it does work!

Maybe it's possible to get the "max_allowed_packet" variable in any way (via a SELECT?) from the mySQL server. I nice idea would be to check this size with the maximum upload size from PHP. If the "max_allowed_packet" value is lower, warn the user/administrator.

SHOW VARIABLES LIKE 'max_allowed_packet';

Thank you for taking the time to report a problem with mantis.

Since this problem report was originally made, a number of releases have occurred.
It appears that this issue has either been fixed, or may not be a relevant report for the current release.

Unfortunately you are not using the latest version and the problem might already be fixed. Please download the latest release from http://www.mantisbt.org/download.php

If you are able to reproduce this bug in the current release, or have some more information on how this feature could be improved in the current release. Please either change the mantis version on this bug report
to the version you tested and change the status back to "Open", or open a new issue report wit more information.

Again, thank you for your bug report and your continued support.
Paul