Well in the end I was banned five times, each time straight after making a perfectly ordinary game related post and each ban for a progressively longer time.
The admin danced about it for a while - the forum admin saying to send in a ticket and then the technical support people saying contact Ivar Marksman - but as each fresh ban occurred they speeded up and finally "white listed" me. The forum admin tendered an apology.
I suppose something in each post triggered the spam algorithm but what it could have been I don't know.
For myself I am glad that the spam posts get removed. How much trouble the defects in the algorithm cause I do not know. Perhaps plarium will address the defects. They should do so; the "banned" banner placed on accounts is pretty clearly defamatory, at least in some legal jurisdictions. Chess.com was sued some time back for mistakenly banning an account and there have been other cases.