Hi, so we have successfully reproduced the issue and there really is a bug. It should be fixed in about 2 or 3 weeks in version 6.12.1, thanks for your patience.
Having the same problem on my HTC Device. Also Avast Cleanup can't be installed. Neither by GooglePlayStore nor by download on the avast.com website. Install fails after downloading getting error -504. The Link in the error message for help/support is un-clickable.
HTC One
HTC Sense Version 6.0
Software Number 7.19.401.51
HTC SDK API Level 6.55
Android 5.0.2
Avast Mobile Security 2018 and also Avast Cleanup were both installed 2 weeks ago. It appeared suddenly with an "Unable to update - Error -504" message via Google PlayStore. Deinstalling ande reinstalls not successful.
Please provide a fix/patch/workaround for HTC Users. ASAP. Thank you, developers.
Kaspersky's similar free product is installed without problems, but is not my first choice. But it solves temporarely this "buggy" problem for me on my device.