OK, here's the problem. I've searched these forums high and low for an solution to this issue but haven't found an answer. I've also just found out some more info:
When visitors to my XOOPS site (2.0.13.2 (downgraded from 2.2)) try to register they just see a blank page at /register.php when they hit the submit button. No details are entered into the database and as a result I've gone a month without new members.
PHP debug gives up no new error messages and I've uploaded fresh copies of register.php just in case it was corrupted. Still no joy.
I have noticed however, that I can add new users through the admin panel when I'm logged in as the administrator. I've logged in as the new user and it seems to work fine.
Does this mean that my database is ok (as it IS accepting new members). This would lead me to believe that there's either some kind of problem with permission settings or some file error somewhere on the site e.g. php error not detected by debug mode?
Please please somebody help me out here. After a month of this I'm loosing the will to live!!!!
ANY hints / tips are much appreciated.

this might not help, but you should check your templates. After I downgraded, I got weird blankness on random pages until i switched to a backup template set.

Thanks Mantooth, You may just have helped me stumble on something here.
I just went to check on my template sets and noticed that while I'm setup to use cloned templates, there are no cloned templates there to use! Strange...
I also don't seem to have the option to clone the default set at all - I'll have to work that one out.
In the meantime though I've set the site to use the default set and the same problem persists - I get the feeling that the site was using the default templates despite being set to use cloned ones (that don't seem to exist).
So there's definetely something strange going on there and I'm off now to get to the bottom of it.

try reuploading "includes/registerform.php" that could be the problem

Thanks Bandit-X Quote:
hmm - I just tried it there and no change - you mean the 'include' directory?? (without the 's').
I am still trying to sort out a few problems with the template sets and preferences though. I reuploaded a fresh copy of the system module and while I now have the option to clone my templates I can't access the preferences to set which set I want to use! Also the system configuration area shows '_MD_AM_COMMENTS' etc... I'm still in the middle of working on this area though and I'm hopeful I'll have sorted later today.
In the mean time any other advise or tips really is appreciated. Thanks V much guys.