PS: Set-ADAccountPassword - Complexity Exception

I've got problems with setting new passwords via Powershell on Server 2012 R2:

Set-ADAccountPassword -Identity testuser48 
-OldPassword (ConvertTo-SecureString -AsPlainText "HelloPassword123#" -Force)
-NewPassword (ConvertTo-SecureString -AsPlainText "April456#@123" -Force)

I tried many different passwords but there's always an ADPasswordComplexityException.

FullyQualifiedErrorId : ActiveDirectoryServer:1325,Microsoft.ActiveDirectory.Management.Commands.SetADAccountPasword

Are there any other things I could try? All password complexity rules (incl. length, ...) are disabled.