Meanwhile I've fixed the problem.
To resolve the issue, I did the following actions:
- Uninstalled UltraEdit 17
- Manually removed all UltraEdit stuff from the registry (CLASSES_ROOT & CURRENT_USER)
- Switched OFF UAC
- Installed again UltraEdit 17
- Performed file type registration with UltraEdit 17 (startup dialog)
- Tried to open a .txt file via double click in Explorer -> it worked
- Switched ON UAC again
- Tried to open a .txt file via double click in Explorer -> it worked -->
I'm sure that it is a problem from UltraEdit. Something might be wrong when UAC is enabled during install & file type registration or when installing UltraEdit 17 without manually un-installing UltraEdit16 before.
BTW: currently I use UltraEdit without DDE. Opening new edit views via double click from explorer to a .txt file is much slower as with DDE but I'm happy that the annoying message box is gone.