I was having exactly the same problems opening encrypted files. It was Bulldog and they are aware of the issue, they gave me a work around fix, Hope it helps it worked for me.

1. Open BullGuard and from the upper menu access Settings > Advanced.
2. Move to the Antivirus section and turn off the Antivirus Protection.
3. Go back to Settings > Antivirus > Tuning.
4. Place a checkmark for "Skip files/folders" and on "Skip Specific Processes".
5. Click on the "files/folders" link and then on the [+] button to add the path of the file to the scanning exceptions and for the Office folder then confirm with OK.
6. Click on the "Process" link and then add the software launcher, word.exe.
7. Go up to Antivirus and re-enable the Antivirus Protection and check if the problem persists.