This warning is normal and usually issued in these circumstances, the first being the most common cause:

1. After DEVONthink has terminated abnormally, e.g., by crash or force quit.
2. When trying to access a database already opened by another person. This includes a database open in another logged in account on the Mac. DEVONthink is ​not ​made for multi-user simultaneous access.
3. If you were trying to open multiple copies of the same database. This is usually caused by copying and renaming a .dtBase2 file in the Finder, and should not be done.

Press Continue . DEVONthink will do a checkup to ensure the database is healthy and report if there are problems.

If you feel sync isn’t working correctly, these are the first things to check:

• In DEVONthink on the Mac, see what is reported in Window > Log. Then check the Troubleshooting > Sync Issues section of the built-in help.
• In DEVONthink To Go see if there is a triangle on the sync icon in the bottom toolbar of the database screen. If so, tap it to see what is reported. Then check the ? > Help > Sync > Troubleshooting section of the built-in help.

If you need further assistance than you can glean from the help or the downloadable manual, you will at least have some information to pass along to the support team.