I’m using the free version of Avast 17.6.2310 on Windows 8.1 64 bit and I just tried scheduling full scan (all hard drives) , then scheduled another for memory scanning and both worked ok. Don’t try changing the options on already scheduled scan, it won’t work. Always create new custom scan with new options.
Also, it could be OS related issue, maybe other win10 users can try scheduling custom scan and relay their results here to help you out.