The digital signature for this file couldn't be verified

Hi, I was getting a featured update bringing my windows 10 20h2 to the new version 21h1. The update went well and I worked for some days already when I suddenly got a virus (Trojan:Win32/Tnega!ml) on my system. Because it seems to be too complicated to get rid of this, I decided to restore a backup I have made successfully with Aomei Backupper v 5.6 just before the update to the new version 21h1. Everything went well with the restore, but after booting up my system I've got the message
Recovery
Your PC/Device needs to be repaired.
The digital signature for this file couldn't be verified.
Error code: 0xc0000428
You'll need to use recovery tools. If you don't have any installation media (like a disc or USB device), contact your PC administrator or PC/Device manufacturer.

Processing a startup repair with a windows installation medium was not successful.
The log file said beside others:

"Root cause found.
A recently serviced boot binary is corrupt"

Also trying to repair the mbr using the following instructions I've been reading in the internet

C:
cd boot
attrib bcd -s -h –r
bootrec /rebuildbcd

did not help.

I do not know if this would make any sense with my system. The motherboard of my desktop is a MSI B406 Tomahawk which is running with a UEFI only - meaning there is no CSM mode. The C partition which was hosting Windrose 10 was created as GPT. I guess this means there is no mbr which could be rebuild or repaired with the above mentioned instructions.

Last may be helpful information:
The backup I was using to restore was created when all updates at this time has been finished. There was no pending update left.

I really don't know what to do and any help is urgently welcome!