> I’ve got a problem installing FreeAdHocUDF on FB2.5.2.26540.

...

> I have palced all dll’s as instructed and renamed main dll as suggested.

...

> Install sql: (need only this UDF)
> DECLARE EXTERNAL FUNCTION F_COUNTWEEKDAYS

You need only this single function from FreeAdHocUDF ?
Then write your own udf without all those crazy dependencies.

I guess all is caused by calling isc_decode_date() from firebird
client library. If you write your own udf in Delphi or FreePascal,
then converting date from firebird format into TDateType
is as simple as
D := F - 15018;

Ivan