If the wallet is 1y+ old I'd recommend to recover the balance using your seed. Backup format changed since, but keep it just in case.
To do a quick try, enter the seed phrase into recovery of wallet.cashu.me - it automates some steps at the cost of less flexibility for complex recoveries, but usually worth to touch.
If it wan't brimg it back, you'll need to figure out which of the older keysets was used to issue your ecash and select it in Minibits wallet recovery tool - if you had long wallet history this may take some time to complete.
You still can: 1. Wait for the fix and try to import the backup file again, later recover wallet address only using the seed.
2. Use your seed to re-create the balance doing full scale balance recovery (can take longer with wallet with a long history, that's why backup option exists)
Sorry there is a bug in the recovery from backup. Will work on the fix.
In the meantime, if you have old device and app still on, move your balance to another ecash wallet, e.g. web based cashu.me and then back to new device.
If you had your own address, use the seed to recover address only (no need to go through full balance recovery).