I assume you are running something earlier than 0.75?
There have been a number of
security issues identified and patched, which you have apparently not done. Prior to the upgrade of the site, some of these were still living here, but I do not think they are available at this point.
In addition, you may be using
3rd party modules that have big holes.
PNphpBB2 unpatched comes to mind.
Also, you will find it is hard to get much help with problems with older versions - most of the folks who provide help here no longer have the old versions available to check/test.
I recommend you:
1. Back up your files and database ASAP. (To restore from when you fall prey to a security flaw or if you have troulbe during the upgrade process.)
2. Look at the modules you currently have and identify any that will be a problem related to the upgrade. That is, any that don't work with .762 and for which there are no current versions or compatible replacements.
3. Upgrade to 0.762 soon
There may be some folks out there who can list the most important security upgrades in the last 3 years, but not I.
--
Peace
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The commonest cause of problems is solutions.