I have the same problem. Did you fix it?
EDIT: I fixed it!
Open super user bash
sudo bash
Go to the NextCloud folder:
cd /var/www/nextcloud
Run the following command:
sudo -u www-data php occ db:add-missing-indices
And finally the prompt should show something like this:
Check indices of the share table.
Adding additional parent index to the share table, this can take some time…
Share table updated successfully.
Adding additional mtime index to the filecache table, this can take some time…
Filecache table updated successfully.
And thats all, the problem should being fixed