Just uploaded a new Beta of au3stripper which should fix the issue when using /rm.
Please be aware that I also renamed the new Directive for clarity to:
#Au3Stripper_Ignore_Errors_in_Func=__DebugMsgBox_*
I have also added the original FuncName to the warning in case it isn't ignored so it is easier to figure the error out:
-### StripOnly/StripFunc Error: Found DllCallbackRegister() statement using unsolvable Func, which will/could lead to removal of Funcs that are used by this Function.
>### current Func: _0 Original_Func_Name:__tttt_messageboxw
D:\Users\jvdza\Downloads\OnDebugMsgBox(1.2020.5.25)\test.au3(21,1): Warning for line:Local $hHandle = DllCallbackRegister($vNewFunction, $sRet, $sParams)
Give that a spin and see ig that also works for you.
Jos