I have quite a few macros and userforms etc set up in Outlook. One of the userforms is set to load each time a mail item is added to the sent item folder. This has worked for about a year but at somepoint in the last few months each time I load outlook and send an email that should then trigger the userform to load, instead I get a message that VBAProject.OTM cannot be found. If I load Outlook and before sending an email run any of the other macros they work fine and if I then send an email the userform opens as intended. Seems very odd and wondering if there has been any recent either windows or office updates that could be impacting on this working properly. There has been no changes to the code in the last 12 months.
There have been some security updates but they should affect it all the time. Are you using a signed macro with security set to only allow signed macros? I'm thinking the signing doesn't kick in immediately.