AirGap uses the aeternity infrastructure to get balances, transaction history and broadcast transactions. In the last couple of weeks, that infrastructure has repeatedly experienced issues. I assume the https://mainnet.aeternal.io/ is behind a load balancer, because sometimes it works, sometimes there is a timeout and sometimes I even get certificate issues. I have reported this in the telegram about a month ago and was told that it is being worked on.
We can’t really do anything about this from our side. We will reach out to the Aeternity team again to get this fixed ASAP. As soon as the issues are resolved, it will immediately work again in AirGap.
In the meantime, try restarting the app (to refresh your balance), or try to repeatedly send a transaction until it works.
An example URL where you can see the behaviour in the browser (refresh it a couple of times to see the different responses): https://mainnet.aeternal.io/v2/accounts/ak_dArxCkAsk1mZB1L9CX3cdz1GDN4hN84L3Q8dMLHN4v8cU85TF