You do not have permission to open the document “256x256_OSX.png”. after the update to Big Sur I am unable to open some files. I have tried to reset permissions in Finder/Get Info but to no avail

I am running a MBP intel with macOS 11.6.2 platform and 2 Thunderbolt ports. I travel much and sometimes Wifi is not available and I have to use the ethernet cable. Before upgrading to Big Sur it was automatically connecting and working well. But now the thunderbolt bridge does not connect although when I look into About this Mac, System Report, it says connected. When I switch the cable to the other port, it also reports 'connected' to the other port in the System Report. But, I have no internet connection because in the System Preferences-Network panel it reports: Thunderbolt Bridge not connected. How should I configure this? Could any one help me out?

My external ssd doesn't show up on the desktop. It is visible in disk utility but it has become unwritable. I can't repair, split or erase the disk' Does anyone have an advise for this issue?

Has any one an idea of how to reinstall AHT in Big Sur?