How exactly are you trying to open the .c files stored in iCloud Drive?

If you use the "Open…" command in Textastic, you should be able to open the files.

If you have multiple files in the same iCloud Drive folder, you can use the "Add External Folder" command in Textastic to add the iCloud Drive folder as an external folder to Textastic.

Please have a look at the following topics in the manual for details:

https://www.textasticapp.com/v9/manual/integration_other_apps/external_files_folders.html

https://www.textasticapp.com/v9/manual/managing_files/local_files_icloud.html

Thanks, I'll look into it.

You should be able to open individual files from the iSH file provider using the "Open…" command in Textastic.

To be able to iSH folders as an external folder to Textastic, this feature would have to be added to iSH's file provider extension (similar to how this is implemented in the "Working Copy" and "Secure ShellFish" apps) - Textastic already supports this. 

This would be totally possible, but would require a major restructuring of the app so that the same file can be displayed and edited in multiple editors simultaneously. Currently each editor works on its own in-memory copy of the file. Maybe I'll implement that in the future.

I'm not sure if I understand correctly. Are those files stored on a server and you don't see them when connecting to the server with Textastic's SSH client?

Yes, that's a good idea. iPadOS 15 seems to add an official way to add tooltips using the new UIToolTipInteraction API and I plan to add it to Textastic along with other improvements for iOS 15.

Maybe there is a tweak installed that causes problems. I'm running out of other ideas.

Does changing to German do anything?

Are you running the latest iOS version? No jailbreak?