Maybe they by default work now on (Native)Segwit, not on old 1xxxx format? No idea.
Did you protect wallet with PIN?
Anyway, they say you may manually restore private keys from backup file (scroll to the bottom):
https://github.com/bitcoin-wallet/bitcoin-wallet/blob/master/wallet/README.recover.md
You may try to use their tool (preferably being offline).
As you receive WIF, you may probably easily import it to any other client, mobile or desktop.
Thank you, I tried it according to the instructions, I got to the DECRYPTING paragraph.
I have Windows, I have installed Ubuntu, but I do not know how to enter the path to the file in the command line :-(