I've had the same problem with this game on Facebook for a week now. I tried it on multiple computers, multiple browsers and multiple operating systems and they all give me the same problem:
When i start Monster Legends it loads all right, it gives me the screen for the May daily bonus. I click on Claim, i see that i get 1 gem and then a split second later the gem screen with claim on it goes away, only to give me the message: We're experiencing problems with our servers. please check back later.
The 1 gem reward is for may 2nd i believe, so i'm thinking somehow, my account has the daily reward stuck and i can't claim anything.
The game works OK on mobile though, but there i have other kinds of issues like Dungeons being far too high level that weren't present on the Facebook version.