I just updated RL "cache cleaner" to v8.0.1
then I clicked on its cache cleaning button in the back end.
Immediately after that my start page* was broken -> showing the permanent error:
"0 No valid image was loaded."
(Back end still working)

then I disabled the RL plugin "Modals" v11.9.3 pro,
and the start page* comes back to life
(of course without all my modals pics within the articles)

That is still the state now:
- enabling the plugin "Modals" -> start page* is broken
- disabling the plugin "Modals" -> start page* works (but without the modals pics)

What can I do ?

(using Joomla! 3.10.3)

*(some other sub-pages are working correctly / some not)

should I try out
the development version of "cache cleaner"
or development version of "modals" ?

I tried Modals 11.9.4-dev2168482 PRO
cleared all caches
but it's still the same error

I give back end access to you:

Yes

hmmm...

that's really bad news,
it's a live - productive site (and I can not get it back to work),
and the PHP is still the same than 3 hours before,
the only thing I changed is that I upgraded the "cache cleaner" to v8.0.1

I've got a clone site (just for testing) on the same server (same PHP) but with a different template,
it still works fine with Modals 11.9.3 PRO and cache cleaner 8.0.0
I am afraid to upgrade to cache cleaner 8.0.1 there -> maybe it happens the same there too

I installed 11.9.4-dev2168578 PRO
but it's still the same error

(cache cleaner is still v8.0.1 free)

I don't know if I understand you right (or if I understand what to do exactly):
- how can I "delete" (or uninstall) the library ? (in the back end or via FTP ? -> which file then ?)
- which system plugin folders do I have to delete ? (via FTP ?)
- where do I get the "old" Versions of the library and of the chache cleaner plugin (8.0.0) then ?

maybe this helps (?):
if I switch the template (to default protostar)
I can see the correct prontpage until I first Press the cache cleaner button in the back-end

so the order is:
1.) modals plugin is disabled
2.) switch the template to default "protostar"
3.) reload the Frontend (Ctrl+F5)
4.) enable modals plugin
5.) see the front end ("protostar"-style) working with correct modals pics (!)
6.) press the cache cleaner button in the back-end
7.) error shows up in frontend: "0 No valid image was loaded"
8.) disable the modals plugin again
9.) switch back to my normal template
10.) reload the Frontend (Ctrl+F5)
11.) enable modals plugin
12.) see the front end with my normal template working with correct modals pics (!)
13.) press the cache cleaner button in the back-end
14.) error shows up in frontend: "0 No valid image was loaded"

-> goto 1.)

yes it works now,
as always: thank you very much