There are a million and one settings that affect character encodings, nailing down which one is causing the issue is a case of trial and error.
You say that both forums have the same modifications, but they both look very different, using different CSS, side panels, etc. These modifications are not the same and I suspect that the more likely reason is one of these is causing the issue.
We have carried out 100's of upgrades of version 9 to version 10 of forums hosting our own own network and not see any character encoding issues like this, so suspect that it is your modifications that are causing the issue.