Nextcloud version (eg, 10.0.2): 11
Operating system and version (eg, Ubuntu 16.04): ubuntu server 16.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 5.6): php 7.0.15
Is this the first time you’ve seen this error?: yes
Can you reliably replicate it? (If so, please outline steps): yes
The issue you are facing:
Each time I am trying to upload a .java file, nextcloud is returning an error.
Via the windows client, I have a not found on the server error, and via the web interface, I have “the target folder “folderdestination” does not exist any more”. But the folder does exist… I can upload files into it and create folders, but not upload .java files.
The output of your Nextcloud log in Admin > Logging:
Nothing in the log
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.