invalid request BLR at offset 163

I have the following error in the Firebird Database. version 2.5.2

invalid request BLR at offset 163 
function F_ENCODEDATE is not defined     
module name or entrypoint could not be found

Error while parsing procedure GETMONTHSBYYEAR's BLR

Until last week everything was functioning correctly. This UDF exists on the disk. How can I debug this problem? Anyone can help me to sort out this problem.

PS: What I did so far to fix:

• Backup / restore - no result. (any structure problem is fixed in my opinion after a BK/Restore).
• Comment all dependencies, drop UDF function, recreate again - no result.

Potential problems could be that he UDF dll is inaccessible for the server (eg due to permissions, or the UDF restriction config in firebird.conf), or you have installed a 64 bit version of Firebird and your UDF is 32 bit (or vice versa), so Firebird cannot load the DLL.