I translate the notice as follow: Please select a value that is not less than 1. Such notice doesn't comes from i-MSCP. Please give use more info by following our reporting rules: Reporting rules - Reminder
Soll ich diese auflisten?
No but first, disable all of them and give a new try.
This message ist NOT from i-MSCP and even not from this browser-tab, the message is not shown in i-MSCP content, but from your browser "back" button.
Please switch through your open browser tabs and see, where something is marked red. Especially if you have a burning board where you are administrator of.
this might be of help https://community.woltlab.com/…/251906-forum-bearbeiten/ and this https://www.enigma-forum.de/thread/5543-limit-thread-view/
happy to google it for you 
actually I noticed that too, If the quota on the "normal email" tab is set to 0, and zero is not allowed (at least I suppose that in this case my customer was not allowed to set an unlimited quota) it complains for forwarding mails as well, although this setting should not matter for forwarding mails
So, I double-checked it. The weird thing is, that the reseller and the customer even do have an unlimited email quota.
As soon as I changed the value on the normal-email tab to any value > 1, it works for forwarding mail as well