I have seen so many post about this and anyone hasn't got a correct answer.
Here it is, maybe some years later
What is custom.hpk?
Contains all custom sprays used for the users who play on your server.
Why appears this?
This happens because you delete custom.hpk from server directory, normally because you don't want custom sprays being used by players.
and your server directory hasn't got the required permissions to write a new empty file.
How to disable custom sprays?
Just add at server.cfg (and restart your server) or add at amxx.cfg:
See info about cvars here: http://txdv.github.io/cstrike-cvarlist/
Also here: https://help.steampowered.com/fr/faq...A111-0949-C6C2
How to fix this spam error at console?
Just download the empty file 'custom.hpk' attached and upload inside cstrike folder.
Don't create by your own a new file called custom.hpk because you will have another error for unknown HPK format.