So small files still work?
And if large files worked before, what has changed since?
Having a bit more information, especially the logs could be helpful. What might play a role is that the temporary folder is not large enough and more filled up compared to before.
A few starting points from the documentation:
https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/big_file_upload_configuration.html
(ideally, the different causes should give you slightly different errors.)