Please proceed as described by @leocg above.

For the sake of completeness, the following remarks:

I just had a look at the extensions and realised that now I only have adblocker, and on the list of Opera's extensions there is no ghostery nor the to-read-sites!!??

Something must have gone wrong loading these extensions.

You can get Ghostery for Opera here. I also used this extension for a while, but I don't like its strategy anymore.

The To-Read sites add-on is unknown to me so far. Is it this one?

As an Adblocker you have probably used Adblock Plus, but Opera has a wonderful, very fast, reliable own Adblocker integrated, as you have already noticed.

And is there a possibility to go back to a previous version (just for a short time so that I have access to my to-read-sites and store them?

You can download all previous Opera desktop versions from here. If the current program version should be unable to read the data from your backup, but an older version can, this might be helpful.

And what about the option to set back Opera to the original state?

This would reset Opera to its default settings, and is not what you want.

In case @leocg has no objections: If the actually obviously not loaded To-Read sites extension should manage your missing start page entries (and not Opera), it might be right to install this extension first instead of restoring the data from the backup first. Maybe you will have access to all missing data afterwards.

Good luck!

Please don't forget to back up your current configuration so that you can restore it if necessary.

So you have a backup? Well, lots of things may have changed since 2016 but you can try using it to get your old speed dial back.

According to leocg, there seems unfortunately nothing that can be done anymore (as long as you don't have any backup, of course). I am sorry.

Anyway, i don't think it's relevant anymore.

I can't find Opera's profile folder

Choose from Opera's menu "About Opera". In the section "Paths" you can see the location of your profile folder. You can also mark this path, copy it via CTRL+C, open Windows Explorer, press F4 and paste via CTRL+V the path from the clipboard.

I suggest to make a backup of your profile folder.

I don't know whether I use the Presto or the Chromium version.

Your Opera version is Chromium-based (v55.0.2994.56, however, that wasn't exactly the question, josef. The question was from which version you updated to v55).

EDIT: BTW, there is a similar thread, in which @burnout426 contributes. Maybe he is so kind to have a look into your issue too, because his (and also @leocg's of course) expertise is much broader than mine.

I am still not quite sure which exact program version you are using, and most importantly, when exactly this loss of working add-ons (such as Ghostery) or your Speed Dials occurred (for the first time): When you updated from an old Presto version, or does this happen again and again with current Chromium versions? Do you use Opera's integrated adblocker or a third party extension?

Please tell us at least your current Opera version (and OS). The more precise the data, the higher the chances of successful help.