While debugging "cannot add new friend" I cleaned my lists.
Now I have only 4 on my Friends List, 0 on my Ignore List and 1 on Watch List.

If I try to invite a new friend I still get this message:

<Unable to send the friend request as one of you are past the maximum allowed number of friends. These limits can be increased by the premium membership program. For more details see the premium settings page on the website>


Are there any suggestions for solutions?

It is likely that the player you are trying to add as a friend has reached his limits. Try adding someone else.

I randomly selected a user and at this user the friend request was sent. So it seems that the friend list is really too long for the previously selected user.
Thank you.

You are welcome.