Timo Club Blocks Coin Purchases With "Invalid Payment" Error

I haven't been able to purchase coins on the Timo Club app for about 3 days now. When I get to the withdrawal or coin purchase step in the app, I get an "invalid payment" warning immediately — before I even reach the confirmation step — and the transaction never completes. I use Paycell as my payment method, and because of this, I can't buy coins, which means I can't use the app the way it's intended.
Despite trying repeatedly to reach Timo Club customer service about this, I haven't been able to get through to them at all, so I can't find out what the problem is or come up with any kind of solution. I want this situation resolved, and I also want the issue causing the "invalid payment" error on Paycell transactions to be identified and fixed.