It suddenly becomes impossible to transparently log in to Parallels Plesk Panel from the Parallels Plesk Expand interface.
Most likely, Parallels Plesk Expand (PPE) compatibility was broken after installation of the latest MUs, which do not contain PPE compatibility patches.
The issue will be fixed in a future MU for which a new Knowledgebase article will be created. You may subscribe to new articles on Parallels Plesk Panel products if you wish to receive notifications about their creation.
Two workarounds are possible:
a. Disable Micro-Updates on 9.5.5 servers registered in PPE.
b. Reapply this patch to all 9.5.5 servers registered in PPE.