Commented by: @pavel-zotov

Problem seems to be related to db-level trigger per se rather than attempt to drop it.

This:

shell del C:\MIX\firebird\QA\fbt-repo\tmp\trgtest.fdb 2>nul;
create database 'localhost/3333:C:\MIX\firebird\QA\fbt-repo\tmp\trgtest.fdb' user 'SYSDBA' password 'masterkey';
show version;

set bail on;
set list on;
set term ^;
execute block as
begin
begin
rdb$set_context('USER_SESSION','POINT_1','1');
execute statement 'drop trigger trg_tx_start';
when any do begin end
end
rdb$set_context('USER_SESSION','POINT_1','2');

end
^
set term ;^
commit;

recreate table tlog (id integer, MSG varchar(100));

set term ^;
create trigger trg_tx_start inactive on TRANSACTION START as
begin
execute statement ('insert into tlog(id, msg) values(?, ?)') (current_transaction,'Tx start');
end
^
set term ;^
commit;

alter trigger trg_tx_start active;
commit;

set autoddl off;

set echo on;

select * from tlog; ---------------- 1
rollback;

select * from tlog; ---------------- 2
commit;

select * from tlog; ---------------- 3

-- does not contain DROP TRIGGER statement but also failed.
Tried on:

ISQL Version: WI-V3.0.3.32718 Firebird 3.0
Server version:
Firebird/Windows/Intel/i386 (access method), version "WI-V3.0.3.32718 Firebird 3.0"
Firebird/Windows/Intel/i386 (remote server), version "WI-V3.0.3.32718 Firebird 3.0/tcp (csprog)/P15"
Firebird/Windows/Intel/i386 (remote interface), version "WI-V3.0.3.32718 Firebird 3.0/tcp (csprog)/P15"
on disk structure version 12.0

Console output:

select * from tlog; ---------------- 1

ID 12
MSG Tx start

ID 13
MSG Tx start

rollback;
Statement failed, SQLSTATE = 08006
Error reading data from the connection.

firebird.conf:

RemoteServicePort = 3333
Servermode = Super
IpcName = fb30ss

LockHashSlots = 22111
TempCacheLimit = 2147483647

#⁠ actual only in Win, FW = OFF
MaxUnflushedWrites = -1
MaxUnflushedWriteTime = -1

#⁠ExternalFileAccess = Restrict C:\MIX\firebird
TempDirectories = c:\temp
MaxUserTraceLogSize = 99999

FileSystemCacheThreshold = 65536K

AuthServer = Legacy_Auth,Srp
AuthClient = Legacy_Auth,Srp,Win_Sspi
WireCrypt = Disabled
UserManager = Legacy_UserManager
DeadlockTimeout = 1
UdfAccess = Restrict UDF;C:\WINNT\system32

Commented by: @pavel-zotov

Problem does exist in _all_ 4.0.x builds since 4.0.0.0032371-0_Win32.7z (03-mar-2016) - console output finishes in all cases with the same text:

ID 13
MSG Tx start

rollback;
Statement failed, SQLSTATE = 08006
Error reading data from the connection.

As of 3.0.x - it looks "much better".
Particularly, 3.0.0.31472-0_Win32.zip (10-dec-2014) it hangs with producing following output:

ID 13
MSG Tx start

rollback;

select * from tlog; ---------------- 2

Commented by: @pavel-zotov

Attached file "crash-on-drop-db-level-trigger-b.7z " contains .sql script that reproduces crash and its stack trace for simple case:

===
alter trigger trg_tx_start active;
commit;

set autoddl off;

set echo on;

select * from tlog; ---------------- 1
rollback;
quit;