There wasn’t mention of anything related to Nextcloud in the release notes for this release. Was it? I am so sorry to tell you this, but we are not going to address all issues at once in releases. That’s not even possible. And we are not going to increase the priority for the issue, which you have already reported to us if you are going to spam every thread. 
What I can reveal now is that we will release another Turris OS release in such a short time, where we fixed the regression caused in Turris OS 6.1.0 related to the LAN page and nothing else.
If you want to look under the hood, please check the response from my colleague, which he replied to you here:
TL;DR: The issue is not confirmed. The issue it lacks some details which would help us to reproduce it.
Ok, I will take it from the end.
user:
Not every process is running under the root and that’s the case with syncthing. The process has its own user under which is started.
credentials / how to log in to syncthing:
For the syncthing, there are two ways, how you can access the own web interface.
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The easiest way is to access http://yourrouterip/syncthing/ and it will ask you for your root credentials. Similarly, as it is done for Transmission WebApps. We will later convert this to Turris Auth integration, but that’s will most likely be done in one of our feature releases to split those changes into small releases instead of one huge update.
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If you want to avoid our WebApps integration, then it is just fine, but you need to change the gui_address in the config to get this working.
What you should do is to configure the syncthing interface once accessed.
the service is not automatically activated:
You want to configure the service first and then enable it. That’s the OpenWrt behavior for this package: packages/utils/syncthing/files/syncthing.conf at ce0679b61a5927ffdbc8a1505ce52756c6c394d4 · openwrt/packages · GitHub