Finally, I uninstalled avast and then re-installed it for another trial. The version for free avast anitvirus is 19.4.2374 - build 19.4.4318.451. For virus definition, it is 190412-8.
After carrying out full virus scan and checking corresponding report, the result is shown below(direct copy from the report)
Avast Scan Report
* This file is generated automatically
*
* Scan name: Full Virus Scan
* Started on: Saturday, April 13, 2019 3:28:42 PM
* VPS: 190412-8, 2019/04/12
*
Infected files: 0
Total files: 530369
Total folders: 43439
Total size: 84.8 GB
No file is reported as 'unable to scan'. Therefore,'Disk 0 Master Boot Record can’t be scanned because other process is accessing it. Error code 32' (not exact wordings, but similar meaning) is no longer stated in the report.
However, according to my experience with older versions of Avast, there should be some files that can't be scanned during full-virus scan, therefore the result seems unusual to me.
My assumptions are:
Case 1: Avast can scan those related files now.
Case 2: Older versions allow user to choose what to indicate in the scanning report, but with the newest version, those files that can't be scanned are not shown in report by default. In the main setting page, users can only decide generate report or not.
Case 3: Other possible causes.
If case 1 is true, I appreciate the developmental effort by avast team because more files can be scanned now for a higher security level.
If case 2 is true, it is difficult to determine whether my full virus scan can scan MBR again or not. In previous versions, files not scanned are listed in the report. That's the reason why some times ago, one avast user checked the report and discovered lots of files can't be scanned, thus he/she reported the incident here and let avast identify the problem as quick as they could. This really serves as a 'double checking process' and help making avast better and better.
https://forum.avast.com/index.php?topic=209475.0Let see whether other users experience these 'changes' in their full virus scan report too or it is an unique incident come across by me.