I try this again, maybe more specs will help to get this resolved:
Account: Slot2, Slot3 & Slot4
Date & Time of the bug: started in the beginning of April
Platform: Win10, browser
Bug step-by-step Description: I had 4 active leagues, then they finished one by one without me starting a new one right away. Within a few days, it said that the slot is locked, as can be seen in the screenshot. At first, I could still start a new league even though it said that the slot was locked, but now I can no longer utilise these slots.