I now think it may be a problem caused by NoScript, which I have removed from Firefox as I was having messages about scripts not completing after closing it. Odd why it seemed to affect this forum but other Simple Machines forums not apparently. Anyway I do not now get the error message.
Edit: No it’s back again, so I will wait for the next CPF beta to see if that fixes it. Still only this forum though.
Well I think I have finally solved this problem after the forum has been very slow and getting this error message.
For the benefit of others I use Firefox with Noscript. Removing script permission for google-analytics.com, which had been in my white list, has resulted in the forum pages loading quickly unlike before. This applies to a few other sites I use also.