Is it because of the too long name? Are there examples of the error being observed with short named files?

The file resized to the size as in the error is available via url https://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/Област_Кърджали_-_Община_Кърджали_-_с._Горна_крепост_-_Античен_и_средновековен_археологически_комплекс_Перперикон_-_%281%29.jpg/1280px-thumbnail.jpg

I see that thumbs URLs for e.g. https://commons.wikimedia.org/wiki/File:Angers_-_Église_Notre-Dame-des-Victoires_-_11.jpg follow the pattern as in the in error's URL.

Do long named files have another pattern for thumb URLs?

So there is an (un)documented feature to add /1280px-thumbnail.jpg instead of full filename, and it works?
Good catch, is it documeted somewhere?

I go to https://commons.wikimedia.org/wiki/File:Област_Кърджали_-_Община_Кърджали_-_с._Горна_крепост_-_Античен_и_средновековен_археологически_комплекс_Перперикон_-_(1).jpg , I click "1,280 × 947 pixels" and it works as that uses the URL suffix /1280px-thumbnail.jpg.

Now how to see/get a link that leads to the error page above? Where are those "long" type of URLs used/shown?

I see... so the pattern is inconsistent, because some other File: pages do create a different last part of the URL which is displayed / linked on the File: page. Confusing.

Duplicate of T106517 (the 501 there is a typo, or has changed since). See also the explanation of URL generation there, basically it's a length limit.