error: Incorrect database or cursor name format

Does anyone have got an idea where I can find a solution for the following problem: I am running a 4ge-application since years. The database-tables are situated in some different databases. In order to fetch datas to work with them I use the command "DATABASE xyz" to change from the current database arzt to database xyz and "DATABASE arzt" to come back to the first database. This worked fine. Since some time it does not work any more: the following error-message comes: Program stopped at "menues2.4gl", line number 548. SQL statement error number -354. ISAM error number 0.Incorrect database or cursor name format. On this line 548 the command "DATABASE arzt" is situated. But: This error did not occure at every function, that does this command. There are functions with the same command "DATABASE arzt" and it worked. First I thought, I made some mistake in the menues2.4gl-file. But the erormessage comes in functions, which I did not touch in recent weeks. So I would like to get some information over this errormessage. In Google I found only the information that the curser has to have a certain length and so on... but I did not do any changes with that file, that contains the command "Database arzt". with isql the menu-command "Select Database" does work. With the command "database xyz; database arzt" I can change to xyz and back to arzt. When creating a new program with these commands it works. Only in that first programm this command "DATABASE arzt" is beginning to make problems. But not all at once, succesively the problem is rising. First only in one function now in all functions. It seems that there the mistake is not in the sorce-code (4gl). Could it be that there is something overrun, to many open cursors or tomething like that??????? I am not very keen in INFORMIX basically. Does anyone have got an Idea wher I can find the mistake? Thanks Philipp