Problem

PERIFACT returns an error message "Cannot open database (locked)".

Possible causes

• Database file is locked: This can be done by another installed tool or by an external program such as a virus scanner.
• External virus scanner conflict: Some external virus scanners, such as Norton or Avast, may conflict with PERIFACT and block opening the database. Continue reading below at (3)!

PERIFACT, despite being "big" on the product, is only a small player in the world market. And thus an unknown to many virus scanners. 

Solutions

1. Reboot the PC:

Try reopening PERIFACT after rebooting the PC. This may solve the problem if the database was locked by another user.

2. Disable the external virus scanner (temporarily):

If you use an external virus scanner, such as(!) Norton, Kaspersky or Avast, disable it temporarily and try opening PERIFACT again. If this solves the problem, add PERIFACT to your virus scanner's exceptions.

3. TIP: Use Windows Defender:

Consider switching to Windows Defender, the free built-in virus scanner from Windows. Windows Defender is less taxing on your system and is less likely to conflict with other programs. Thereby, according to tests, WIndows Defender also does a very good job. Other commercial virus scanners are now so large in size and functionality that they often clutter an entire system. Just Google "norton bloatware" (read: junk). Or if you don't want to Google, just click here: https://www.reddit.com/r/pcmasterrace/comments/17blufq/is_norton_antivirus_bloatware/