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This issue is fixed in the next update, version 4.1
Any Troop with the Sin of Maraj Omen Trait may cause the game to freeze at the start of Battle. Usually, when a Battle begins if there are no available matches the Gem board will shuffle the Gems. If a Sin of Maraj troop with an ‘Omen’ trait unlocked activates at the start of the Battle, this will cause the game to freeze as there are no matches available to make and the board will not shuffle.
When the game freezes you cannot select any Gems but are still able to cast a Spell if it is ready.
Fixing the problem
The Gems of War development team are looking into correcting this issue. In the mean time, try using a Troop with the Empowered Trait when versing Sin of Maraj Troops or when using Sin of Maraj Troops. This will allow you to start the Battle with full Mana and get past the freeze.