I’m not sure I have the knowledge to properly answer your question. This current theme is a premium customized theme just for us, which we can’t afford to just drop unfortunately.
I do hope you can find some resolution to this. This forum plugin has been really helpful and we’d really hate to lose it at this point. We did test it through the customization process of this current theme and we didn’t notice any issues until recently. Just not sure what could have thrown a monkey wrench into the mix.
What I can try to do on my end is to clone the site, place it in a temporary domain, then to dissect the existing plugins and see if any of them are possibly causing a conflict with it, rather then the WP theme.
UPDATE: DEACTIVATING EVERY SINGLE PLUGIN EXCEPT SIMPLE-PRESS DID NOT RESOLVE THE ISSUE. I GOT THE EXACT SAME ERROR MESSAGE.