This Database Engine Error message can occur correctly if there is less than 300MB of free space on the drive where the Q++ServerXE2 folder is located and less than 600MB of free space on the disk where the local Q++ folders are located (the minimum is only 300MB in the case of a Data Contributor user).
This message can also occur incorrectly under certain conditions. Usually this occurs if you local C:\ drive is larger than 4GB (Gigabytes) and/or has more than 4GB of free space left on it. Strangely enough, a solution which has been shown to work is to empty the Windows trash can. If that does not work, make sure that you have updated to a version of Q++ from after June 2005 (build 11200 or above), when this problem was solved once and for all.