You can better convince your client to use Phorum 5 after all. Phorum 3 is nor maintained, nor supported properly anymore. Especially the maintenance stop is a serious issue, because new security insights are not implemented in Phorum 3. Not that it is known to be insecure, but your risk is probably higher.
The behaviour you describe is unknown to me, so I'm afraid I have no good place to look at.
The behaviour you describe is unknown to me, so I'm afraid I have no good place to look at.