Not familiar with Altcoins that much but i tried creating multiple wallets, imported their private keys and it did work, i could see the address, until i tried using the mnemonic seed i got from their web-app.
Try typing out the private key on trust wallet  instead of copying first  , if it still doesn't import it  then:
Do it the other way round  , import your private key on their web-app https://guarda.com/app (not a good practice but left with no choice) check if it returns same address , if it does  then try performing a transaction there.. if same error still occurs then  i don't think there's anymore thing you can do (maybe) than waiting for the support response..