A few more thoughts/things you can try:
Does Nextcloud also complain about the + and # characters when you upload those files via browser? If so, it’s likely your server configuration that is causing the issue, either the Nextcloud server config, webserver/reverse proxy config, or perhaps the storage where the Nextcloud data directory is located.
In this case we’d need more information about your server configuration to see if we might be able to identify the cause:
- Web server / reverse proxy configuration
- OS, database, installation method
- Nextcloud configuration (the output of
occ config:list) - Storage configuration
- Apps you have installed (
occ app:list) - Any error messages or warnings from the logs (
nextcloud.log, and maybe webserver logs)
…which are all things that are asked for in the support template, btw. 
If it works via the browser:
Try to completely uninstall the client on a PC, including deleting the Nextcloud folders under c:\Users\user\AppData\Roaming and c:\Users\user\AppData\local, and then re-install the client and set it up again with the latest version.
If it still doen’t work, maybe also have a look at the client log files under %appdata%\Nextcloud. Additionally you can also create a debug log, which should contain more information: Troubleshooting — Nextcloud latest Administration Manual latest documentation
If that still doesn’t help, you could also try temporarily setting the loglevel on the server to 1 or 0.