Report number 2055057992

I just encountered the same problem and can give some further input. The problem seems to occur when switching from a private to public (open or closed) group if the group name is already taken. Here are my steps to reproduction:

1. Go to the create a new group page (https://www.zotero.org/groups/new)
2. Enter "Masters Thesis" as the group name
3. Select "Private Membership" as the group type
4. Click on "Create Group"
You will be redirected to the group settings page.
5. On the group settings page, change the group type to "Public Closed"
6. Click "Save Settings"

You should now see a notification at the top of the page with the error message "Error saving settings". Looking at the network tab of the Chrome Devtools I can also see a POST request to https://www.zotero.org/group/updatesettings?groupID=[…]&settingsType=library which failed with an HTTP status of 400 Bad Request. In request response I can see a better description of the error:

{"success":false,"message":"Name not available"}