Just to add to this thread, we are aware of further cases where Avast will quarantine the Act!.exe file (and potentially Act!.Integration.exe). This is following the 'Avast! Virus Definitions Update – May 21, 2018'.
We have escalated this to Avast and are awaiting their feedback. As Act! is not the only application affected by this recent update, we are hoping that a further update will be made to Avast soon to resolve it. We would recommend that any affected customers also report this via https://support.avast.com, as this can help to accelerate the solution.
Many thanks for your patience as this gets fully resolved.