My NextCloud was running really slow and I found out that the entries in NextCloud under Settings → Security → Devices & Sessions is the reason for that.
After deleting all entries (hundrets of them) it was running smooth again.
Now I look in again and see that emClient already set up 5 entries again for one client.
Why is emClient produces so many entries? emClient should look into that table and use the existing token and not producing a new token everytime you start emClient.