Hi Max,
I've never seen that error before, but I'm going to say it's not worth trying to solve issues with MAPIEx anymore. It is unreliable and I would try to get away from it as soon as possible. 10-foot pole, in my opinion. Haha!
Try moving to the Outlook VBO from BP 6.3+. It works with older BP versions as well. I think it's just not officially supported by BP for older than 6.3. There are actually 2 new VBOs available on the Digital Exchange, one provided by Blue Prism and one provided by NEOOPS: https://digitalexchange.blueprism.com/dx/search?keyword=outlook
2 things I should mention about the Outlook VBO:
Emails may get stuck in the Outbox from time to time, so you may need to figure out a solution for that. (This only occurs sometimes and as far as I can tell only occurs if Outlook is closed when sending mail)
It requires the Interop dll: Microsoft.Office.Interop.Outlook.dll
Dave
Dave Morris, 3Ci at Southern Company