Some animated GIFs corrupted while watermarking

[Nightfoxx]

Basically I have a website with a gallery that uses ImageMagick to watermark it's images.
Everything works perfectly but some (about 40%) of the animated GIFs I upload end up corrupted while watermarking them.
The thumbnails which don't have watermarks are processed fine (they're not animated though).

I don't know what feature in images exactly causes this, first I thought filesize compared to the with and height but that doesn't seem to be it.
All I know right now is that it's with the same images only. I
mages that got watermarked fine the first time will work again fine if I try again.
This goes for batch uploads as well as single uploads. I asked my host if it's some sort of fault on their part but they forwarded me here.

[fmw42]

Please provide your command line, version of IM and platform and if possible links to a small animation and watermark file for testing that fails.