Hello Sven,
For the 1st one: SEBLOD 3.13 took care of the B/C issue introduced by Joomla! 3.8, but there was a slight issue on our side only when more than one SEBLOD Admin modules were published... so I guess that's why you had blank screen on some of your sites.
For the 2nd one: The slight issue for multiple SEBLOD Admin module was fixed in SEBLOD 3.14
For the 3rd one: Currently I am not sure how this could happen. Please let us know.
In any case, feel free to execute one of the 2 methods to disable the module, at the bottom of the blog post. And when you are able to access the back-end I would even suggest to uninstall the SEBLOD Admin module, and to create a (Joomla!) Admin module using the SEBLOD preset (or a custom back-end menu if needed) ... as the SEBLOD Admin module is now deprecated. With SEBLOD 3.13/SEBLOD 3.14 the purpose was (to try) to avoid broken back-end with Joomla! 3.8, but it's gonna be removed from the Core package soon.
Regards,
Saba.