When you cannot enter the universe with your character this is most likely a known issue. You can ask in the Spellborn Reborn Discord tech-support channel for help with solving this issue.

As for the cause of this bug: it remains unknown.
But it is related to the inventory, and most likely a side-effect of the mail attachment bug in which attachments send to another character are received but cannot be collected by the other character. The attached items are returned to the inventory of the sender to the same slot from which it was dragged and dropped to the mail. When another item has been placed in that slot, this, most likely, causes a conflict that prevents the character to enter the universe after login.