couple questions…
it appears you are using caching… have you told w3tc to ignore the forum page for ALL types of caching in use? its not clear from the w3tc message in the footer if that has been done…
we know wishlist member works fine with simple press and tinymce because we use it here.
but we also know that wishlist tags, which I see you using, is known to be an issue… We even worked with the wishlist tags folks on the problem and a fix, but we have yet to see them release it (a bit disappointing)…
I am also seeing wishlist tags trying to interact with the tinymce instantiation… I am guessing that this wishlist plugin has made a mistake in assuming that the wp tinymce editor would only be used in the wp admin for blog posts… but the wp devs made the editor a class such that it could be used anywhere including on the front end… it seems, but cannot be certain, that its failing on the wishlist tags stuff… unfortunately too many wp plugins that deal with tinymce made the mistake of assuming blog posts only and assumed it was in the admin and that certain js or files may be loaded and have not been updated to stay current with wp… they really should be checking if the tinymce they are trying to load to is the specific one they want to target… or give you options or hooks to prevent this…
I also see another wishist plugin something about idcard interacting with tinymce… no clue if problem…
to top that off, the jetpack after the deadline module is trying to affect the tinymce editor too…
long winded, but can you try disabling those plugins and either confirm or rule them out? start with wishlist tags and run a test.. then disable icard and run test… finally the after the deadline module and test…