Firebird3 x86 WI-V3.0.8.33454 (ServerMode = Classic)
Microsoft Windows Server 2012 R2 Standard

When a server process crashes, for example because of exception in an UDF,
the crashing process hangs and at least blocks access to monitoring tables
(MON$...) for other connections.

with BugcheckAbort = 0 Firebird.log contains following:

MORION-NBK Sun Apr 24 11:39:27 2021
The user defined function: UDF_CRASH_SERVER
referencing entrypoint: CrashServer
in module: sysudf.dll
caused the fatal exception: An exception occurred that does
not have a description. Exception number EEDFADE.
This exception will cause the Firebird server
to terminate abnormally.

MORION-NBK Sun Apr 24 11:39:32 2021
Shutting down the server with 1 active connection(s) to 1 database(s), 0 active service(s)

with BugcheckAbort = 1 Firebird.log contains following:

MORION-NBK Sat Apr 24 16:43:42 2021
The user defined function: UDF_CRASH_SERVER
referencing entrypoint: CrashServer
in module: sysudf.dll
caused the fatal exception: An exception occurred that does
not have a description. Exception number EEDFADE.
This exception will cause the Firebird server
to terminate abnormally.

MORION-NBK Sat Apr 24 16:43:42 2021
An exception occurred that does
not have a description. Exception number EEDFADE.
This exception will cause the Firebird server
to terminate abnormally.

The problem can be reproduced on a large database ~2Tb.
On smaller DBs there are no hangs, so I can not give a test DB example.
Does not depend on the number of connections.
Does not hang with BugcheckAbort = 1.

Process stack after hanging:
fb_stack.txt