Just a heads-up regarding this feature of the Forum. I just learnt it's possible for your PM box to be full, and if it is, the Forum flags an attempted "send" to say your PM wasn't sent because your intended receiver's mailbox is full.
I checked mine, and it dated back to 2016 and had 11 pages of PMs. It's a simple matter to clean out a few year's worth, either a page at a time or individually.
New Year- time for a clean-out!