In test environment temporary databases are created and dropped for every new test. But 'drop database' can lead FB to hang if we do that just after 'alter database encrypt | decrypt' without any pause.
It will be good to receive error like 'object in use' instead.

Steps to reproduce using built-in FB 4.x encryption plugin:

1. change dir to your FB 4.x home
2. run there:

copy .\examples\prebuilt\plugins\fbSampleDbCrypt.* .\plugins\*.*

3. open in editor file .\plugins\fbSampleDbCrypt.conf and add there following line:

Auto = yes

4. run following .sql:

set echo on;
set list on;
set bail on;
shell if exist c:\temp\tmp4test.fdb del c:\temp\tmp4test.fdb;
create database 'inet://c:\temp\tmp4test.fdb' user sysdba password 'masterkey';
 
alter database encrypt with "fbSampleDbCrypt" key "Red";
-- commit;
drop database;
quit;

Output will show "drop database" and will hang infinitely since that.
Expected: message that DB can not be dropped (somewhat like "object in use").

Here is URL to dump, stack trace and official 4.0.1 Release (4.0.1.2692):

https://drive.google.com/drive/folders/1hi4R5gdjUL5MW67_0ET3hWMD3pZuTp6C?usp=sharing

PS.

1. same if we try to DEcrypt database
2. removing comment from "-- commit;" does not help. One need to stay in pause for 1..2 seconds in order DROP DATABASE can be performed normally.