UDFs are deprecated in Firebird 4
From Firebird 3 to 4 list of know incompatibilities file :
UDFs are deprecated in v.4. That means that UDFs can’t be used with default configuration (parameter “UdfAccess” set to “None”) and all sample UDF libraries (ib_udf, fbudf) are not distributed any more. Most of functions in that libraries were replaced with builtin analogs in previous versions and therefore already deprecated. A few remaining functions got safe replacement in UDR library “udf_compat“, namely div, frac, dow, sdow, getExactTimestampUTC and isLeapYear. Users who still wish to use UDFs should set “UdfAccess” to “Restrict <path-list>”. If you never used to modify this parameter before path-list is just UDF and resulting line in firebird.conf should be:
UdfAccess = Restrict UDF
Recommended long-term solution is replacing of UDF with UDR.
Ps: Here is the related git commit .