Was your first test done on a virtual machine? No issues here with WEI and avast installed on Windows 10 1809.
No, I encountered it on my physical computer while trying to determine the Windows Experience Index score again after updating GPU drivers. After a couple of unsuccessful attempts, I started looking for the cause. One of my guesses was an antivirus software, so I tried disabling it, performing a clean boot (so it won't run at startup) but all of it was for nothing – WEI still was failing to complete. Ultimately, I uninstalled Avast and, after a reboot, the test did complete without any error.
And that's where I began to perform more tests on VMs to be sure that my theory is correct before reporting it.
Tested Avast Free Antivirus on Windows Vista, 7, 8, 8.1 and 10, and it looks like this is only happening on Windows 7.
Windows Vista™ Home Premium (6.0, Build 6002) Service Pack 2 (6002.vistasp2_gdr.170209-0600):
works fineWindows 7 Professional 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_rtm.101119-1850):
failsWindows 7 Professional 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_ldr_escrow.190320-1700):
failsWindows 8 Pro 32-bit (6.2, Build 9200) (9200.win8_gdr.151112-0600):
works fineWindows 8.1 Pro 32-bit (6.3, Build 9600) (9600.winblue_ltsb_escrow.190313-1806):
works fineWindows 10 Pro 32-bit (10.0, Build 17763) (17763.rs5_release.180914-1434):
works fine