Thank you for coming back to this. 
There was/is a problem with the image slider we use. We were able to get around the problem by putting this into the mobile CSS:
.has-full-page-chat #captioned-gallery {
display: none;
}
Chat works again, but I still have another issue, which also seems to be related to the image slider:
Every time the image slides, the problem can (only can) occur.
For sure, this is a problem for our forum only because the slider is non standard. But something changed with 3.1.0beta5 which was not there before.
Btw Chat is only available for TL1 and up, so it is not visible when visiting the forum and not logging in.