There are two areas for exclusions, the avastUI, Settings, Exclusions that are for on-demand scans and the File System Shield, Expert Settings, Exclusions, the latter is the one you need.

Is this the one you did ?

Secondly you should exclude only files rather folders as that leaves a hole in security.
Note: When using the Browse button it only goes down to folder level accept that. Now open the entry in the exclusions and change the * to \file_name.exe where file_name.exe is the file you want to exclude.

Lastly - Anything that you consider a false positive, shouldn’t just be ignored (avira speak) or excluded from scans (avast), that is shooting the messenger rather than treating the problem.

Confirm by examination (virustotal) that it is indeed an FP, if so send the sample for analysis and correction of the detection signature plus inclusion in a signature update. This helps all avast users.