@Axoid
The error messages and your reply tells me that it's not finding what it's looking for in the campaign XML (db.xml).
The code looks for a standardized xml entries similar to that which are created when items are added from the Players Handbook. So if the entries are custom made, they might also not fit this setup and thus again the regular expression would fail if it's not done similarly.
So it does make sense that removing and re-adding the items would rewrite them into the campaign database and thereby fix the problem.
Spells are written somewhat differently into the database, so it could be that they had the xml needed in place, where as the weapons simply didn't. They again depend on a standarized entry.
Just to make sure, this doesn't happen again each time you start the campaign right, after having deleted and added the items back in the time before? That is you have a permanent fix you figured out?