Damn, just by the fact alone of removing the mint, you deleted your own money. That is the nature of ecash; it lives locally on your device, "offline", as a JSON file. Therefore, never delete a mint until you are certain all JSON files are transferred to a new mint or spent to a lightning wallet/address.

Next time, try spending the 500 sats from the mint (minibits in this case) to a regular lightning wallet (like wallet of satoshi), then delete the mint after payment is confirmed.

This is ecash workflow (UX) problem*, not a mint or app problem as it seems.

*I actually view this as an feature, not a UX bug. This is because the user HOLDS the sats in ecash itself, locally.

If you ask nicely and help the devs a bit, you may be able to get minibits to give you the 500 sats back.

Also, all mints (currently) run on Lightning, therefore, mint accessibility online can go in and out, and that particular server could say drop its internet for say 0.1% of the time.