This error happens on both Kubuntu 18.04 and KDE Neon when trying to log in with the Nextcloud desktop client (nextcloud-client). Fortunately there is an extremely easy solution that I found here.
The error happens when the nextcloud-client tries to log you into your Nextcloud instance. It opens a webpage but instead of asking for your login details (etc) it just says,
Access forbidden. Your login token is invalid or has expired
It isn’t a problem with Nextcloud. What it is is a problem with KDE that can be solved in System Settings by opening the Main Menu, opening System Settings under Applications/Settings and going to Applications/Default Applications.
It will probably look like this,
And you need to change it to open HTTP and HTTPS URL’s in Firefox like this.
Click Apply in the bottom right side and that should fix it and when you try to connect again the process should work perfectly.