Welcome to the forum! 
There is a bug in the session restore functionality of Plasma right now, which yields the symptoms you are describing. Therefore, you must disable starting with a saved session and instead start with an empty session — at least, until the bug is fixed.
After making the changes, you must reboot.
Note: This issue has already been reported here at the forum countless times since the most recent Stable Update. Next time, please search the forum for similar threads before posting.
Also, you may want to subscribe to notifications for the Stable Updates category. Every major update comes with a dedicated announcement thread, the first post of which details the changes, and the second post mentions the gotchas and how to work around them.
The issue you’re having was discussed on the announcement thread.