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Hello -
I was having these exact same problems and after sifting through the mainfile.php (to no avail, by the way) I had one of those "duh" moments. Actually, it was more of a "I can't believe how stupid I am, I must actually have some sort of mental deficiency" moments.
An internet security suite was really getting vigilante on the cookies... blocking every third-party cookie without even alerting me.
So, I fixed that, and that in turn fixed ALL my XOOPS problems. Including not being able to add new clients to the banners area or new banners to existing clients... this fixed them all.
I recommend that every consider this before wasting time staring at code... I'd completely forgotten I had a new security suite on this system.
Cheers.
Mike