Hello everyone... we did make some big upgrades this summer to all the servers, both hardware and operating systems, with latest versions of pretty much everything. The ironic thing about this issue is that it has been caused by updating too much. The forum software is currently only guaranteed to work with older server software versions -- old versions which we passed up in our upgrades (PHP for those of you that know what that means).
Unless other issues pop up, we will probably wait until the makers of the forum software issue an update to fix this issue, rather than going backwards to older versions of the server software.
As you've discovered, all other uploads on the forums are working, so feel free to put an image in your signature in the meantime.