I've worked out what the problem is!
PHP on the server has been upgraded and has sharpened the security... Tapawhotsit requires fairly loose security so I need to try and balance the two...
How's the balancing going?
To be honest I haven't had much time to look at it as I've been working on the new apartment almost 24/7...
...and the last thing I need at the moment is to gub the forum and have to choose between spending time fixing it and meeting my tight schedule next door.
I'll just say that the feedback from the SMF (the forum software) developers on TapaWhotsit is
not very positive...