Yesterday, I wanted to resume my studies with the collection “Radialistas” but I realized, that it disappeared from “My collections”. Now I’m wondering, if there are any restrictions concerning the number of collections I’m working with?!
@ilmelomane - No collections should ever disappear on you. Have you archived all the lessons from that collection already? That may cause it not to appear for you.
Thanks for your response!
No, I have no archived lessons. I have just deleted 3 of my collections which I don’t need any more and 3 of the vanished - among them Radialistas - reappeared.
It seems that 100 (= 4 pages) is the greatest amount of collections that will be shown in “My collections” With all my Quick Imports I surely passed over this limit. 
Yes, that is the case. You can always go to My Lessons and type the collection name into the search box.