Let’s try some steps:

1. Open outlook in safe mode, and try to reproduce issue Run->Outlook /safe>
Notes: It’ll disable most of Add-ins to Vag 15.7 test if the issue is cause by them
2. Please try to clean up the form cache, we may receive the messages which form has been modified some times
a. “Tools” menu->”Options”->”Other” tab->”Advanced Options”
b. “Custom Forms”->Click “Manage Forms”->”Clear Cache”
c. Restart Outlook for testing
d. If above steps won’t work, try to rename the form cache file
e. Quit outlook, search “frmcache.dat” file on the hard drive, and then rename it
f. Start outlook to check issue
Notes: Form cache will be Tech 2 Scanner recreated when next time you use a custom form
3. Please try to recreate the mail profile in case the profile corruption may be the cause
4. Try to change the location of the PST.