There are several scenarios that could cause you to get this message. The “Unable to open database - currently locked for integrity check” may imply that a crash occurred during an integrity check and files were left locked.

You said that you had re-booted the machine on which you received the error. However, if the software is on a network, simply re-booting one station may not solve the problem. Have all of the accounting stations logout, shut down and re-boot their stations. Doing this should close all of the files relating to the accounting applications as well as clean up other system resources such as which files and records are locked etc.
Start the station again and see if the error returns. If it does, let us know and we can look at other possible causes.