It seems the AvastSvc.exe is functioning normally, only GUI is bugged.
I tested on my Vista machine and successfully reproduced the issue, and Avast blocks eicar as usual, concluding AvastSvc itself is working.
I found AvastUI.exe is not signed as usual, and this might be the cause of the problem, because Avast seems blocking access to its service from untrusted modules using self-defense.
If I disable self-defense module via editing avast5.ini file and restart AvastUI.exe, AvastUI shows avast is working.