Desperado said:
Steps to recreate (it is reproducible):
* Make sure you have some bonding scrolls available for application
1) Populate to max your champion slots, if you don't have them already
2) Take an action that reduces their number (sacrifice, rank up, enlighten)
3) Go to items and apply bonding scroll -> error despite step 2 created empty slots:(
4) Restart client
5) Go to items and apply bonding scroll -> works
Thank you for the details!
Champions upgrades do not cause synchronization with the game server. This is why the information about the slots can be outdated.
You can resolve the situation by sending troops to Catacombs, claiming rewards or making purchases at the Market. After these actions, the data should be updated and the Bonds be applied correctly.