I went for a match and decided to create a new rune page at the champion screen and there the problem occured:
* I selected my champion a tried creating a new rune page. The already existing pages showed up, but when I tried creatig a new one the rune window just disappeared; (I still have one editable page available).
* I also tried before selecting the champion but the same as above occured.
It happened a couple of times (still happening).
At Runes Section, everything is fine and I can create and edit rune pages normally.