Since we upgraded from 2.4.8 to 188.8.131.52 about two months ago, we have had at least six instances where an author in (mainland) China was unable to submit a paper.
The usual behaviour is that when they click New Submission, nothing happens except an everlasting progress indicator. In one case I helped an author through Step 1 of the submission process, but they were unable to continue to Step 2. One author tried from multiple computers with the same result.
This has only occurred from China. However, people in China can read the journal contents just fine.
I could never find anything useful in the server/php logs, but I might have missed it. My best guess is that the Chinese firewall is blocking something essential such as googleapis.com. We know that China regularly blocks multiple Google products. I don’t have a good suggestion.
This stackoverflow thread might be relevant. Or might not.