This seems to be a glitch in the macOS version. Files can sometimes get “stuck” so that the clean broom icon doesn’t work and AxCrypt thinks you have the file open for editing somewhere, and thus won’t allow you to decrypt and forget about it since you’d then risk losing your edits.
The work-around for now seems to be to completely exit AxCrypt. and then click the broom. We’ll be fixing this asap.