I thought you had disabled all other plugins previously?

This would then lead to a couple other things to try… on forum – integration – page and permalink, what is the settings for display in loop and display multiple?  I would suggest true and false… see if that does anything…   if not, then perhaps try enabling the wp strict api option…

As I said, I never had all plugins deactivated at the same time, this would have made my Live Site unoperational.

Instead I deactivated them step by step and tried calling SP forum again. On this first round of deactivations and tests, I found no difference after deactivating wpSEO. But as I said, maybe I did not truely deactivate it and made an error here, for whatever reason.

Anyway, I tried the options you mentioned in On/Off with no effect. Also Strict API with no effect.

I will now direct my questions to wpSEO. It seems, as if they are the cause of the weird lock. 

If they or I find out anything of relevance, I come back to you.

well, still could come down to something with simple press… but seo tools sometimes do strange things with content and make odd assumptions (such as its static)…  and frankly, we have not heard of that one before…

can you disable it for the wp page the forum is on?  its unlikely it will work right anyways since each page view of the wp page is dynamic and in most cases, different… plus we include our own seo stuff for the wp page the forum is on…