As you can see after I enter edit mode and close it, the panel renders normally. This happens all the times after I reboot the system or logout of the session and log in again. Any idea how I could solve this bug?
Unfortunately for now this is normal. I would assume there is at least one bug report about it. I’d suggest doing a search for one and adding your information to it.
Hi - looks like you’re running into the following bug: 496862 – Non-floating Panel with "Fit Content" does not take the correct width on plasmashell start
If that matches what you’re experiencing - the screenshots seem to, even though you’re using a different widget on the panel - then I’d at least suggest following the progress of that bug (unless others have any clever workarounds! 
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