Upgrade your fbclient.dll to Firebird 3, and it should log in the same
directory. The reason it stores the log file one directory higher
derives from how the file layout is in Firebird 2.5 and earlier (it has
a \bin directory for executables and DLLs).
The only other option I'm aware of, is building a custom version of
fbclient.dll.
Mark
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Mark Rotteveel