Nextcloud client suddenly asking to login and erase/sync all data

Nextcloud version (eg, 20.0.5): server 25.0.4, client 3.8.1 (Windows)
Operating system and version (eg, Ubuntu 20.04): Windows 10
Apache or nginx version (eg, Apache 2.4.25): 2.4.41-4
PHP version (eg, 7.4): 7.4

The issue you are facing:
I’ve been using nextcloud on my own server and with windows cloud for years. Today after I started my windows nextcloud asked me to log in (never happened before). I logged in and granted access to the client using web. Then the client asked me to select a sync folder - but I already have a sync folder (C:\Users\Martin\Nextcloud) with all data. So instead of the proposed Nextcloud2 folder I selected the original Nextcloud folder.

Then I was presented with few options.

I don’t want to sync anything from the server. I also don’t want to erase any data and I assume ‘keep local data’ means that everything on the server will be replaced with local data. Why can’t I simply keep local data without syncing anything as the data is equal?

Why is this happening? Why can’t Nextcloud client recognize that the folder already exists and simply resume standard operation?

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

Don’t know. It appears as if Nextcloud client suddenly lost settings, but not sync folder.

I’m not sure what you mean by you don’t want it to sync. That’s what the client does. So probably what you want is to keep local data and use whatever sync settings you had before.

As far as the cause, it sounds like something happened on your computer that made the sync client forget it’s settings. That could have been any number of things since it’s a client-side issue.

The sync client stopped working today morning, but I didn’t modify any of the files locally nor on the server since then. In other words my local files are exactly the same as on the server. Therefore I don’t see the need why the client should try to upload nor download any data. I have experience with broken timestamps and duplicate files if I try to sync equal folders using Nextcloud (e.g. if folder’s content match in cloud and on local pc). I want to avoid that. So I’m afraid to do anything.

If I hit ‘Keep local data’ - is it possible that the local data will start syncing to the cloud and override the data in the cloud, or even duplicate it (which is even worse)? Or will it just compare cloud’s files to local files and not sync anything, since there is no difference?