this hasn’t been reported before – for me to be able to troubleshoot the backend, please send me your login and password to email@example.com (no disclosure guaranteed) – and I will examine the issue.
I’ve checked the slider options, here’s why the issue arose: to be saved by the system, every new slide needs to have an image (it is a mandatory field for this option type) – yet in the image upload overlay box the “URL” field was not filled automatically by the system, despite the fact that in 99% of the cases it is. This prevented the slide from having any images attached, and in turn prevented the options panel from saving the new slide. Should you encounter this again (low probability, but may happen), just press the “Datei-URL” button below the “URL” field in the image upload box (see screenshot). To ensure that everything is working fine, I’ve deleted the last slide and created a new one called “Test slide” with the same image as the first one – just so you know.