Symptoms
In Client Manager > Devices tab, while specifying "15.5" in the "Client Version" field it is showing 0 clients, although some of the clients are on 15.5.
Resolution
Parallels Development Team is aware of this issue and working on the resolution.
Workaround
Should you need to list 15.5 clients only, specify the following in "Client Version" filed:
15.5-16.0