Microsoft Defender report - Stop weak cipher usage report

What do you see in weak cipher report details? If this is related to weak encryption (RC4, DES) that AD accounts are using then you would need to look for events related to kerberos protocol (4766-4768).

A fix for that is by going to AD account -> Properties -> Account -> Account options and tick 2 boxes "This account supports Kerberos AES 128/256 bit encryption". This would default account to use AES encryption rather than RC4.