Hello Raimund,
Thanks for sharing your deployment!
In terms of the upgrade to v5.5, it should be painless, however, I always recommend to make backups of both, your WP database and the old plugin directory.
Also, once installed double check all (new!) features and settings.
Your custom CSS, I believe should work as is.
By the way, I did notice that for some reason this field:
<input type="hidden" value="a47e89a95b7cbc13eb8ad87d37d094fb" name="PHPSESSID"/>
is being added to all your FIELDSETS. Which results in the extra empty field above the others...