Updated to 11.1.2256 from 2255.
Got the welcome window again. Close button did not work.
First reboot and it was still there.
Second reboot and it was still there.
Tried third reboot and it has finally gone.
I dumped AvastUI.exe twice and uploaded it to avast FTP.
It contains two files, AvastUI.exe (1).dmp and AvastUI.exe (2).dmp, these are dump files from first and second reboot respectively.
Filename: NON_AvastUI.exe.7z