PREVIOUS (BAD) REPORT (2010/08/26):
"SUMMARY: user edit post error after admin or global mod added attachment"
"DESCRIPTION:
I couldn't confirm if this is already fixed on 3.0.8

To reproduce:
1) a normal (registered) user posts a message
2) after that, an admin or global moderator adds an attachment to that message (for e.g. to convert an outside image link to an attached image)
3) user edits message and then tries to preview or submit
4) "General error! You are not allowed to access this file."

I think this may be somewhat related to a previous fix on 3.0.6 RC 1
http://tracker.phpbb.com/browse/PHPBB3-8727

Best regards,
Nuno Nina"

CORRECT UPDATED REPORT (2010/08/27)!!!!!!!!:

First of all let me apologize, specially to Joas Schilling, for me not testing this intensively...

I've just installed a stock version of phpBB 3.0.7-PL1 and indeed it does not fire this General Error :S

The error we are getting looks like a very odd, unique, and not very easy to reproduce... After searching all over the web and with the very little information we have I wrongly assumed the cause... (I'm really ashamed of)

Please read comments for updated information...