Technically, software doesn't "break". If it works, it continues to work forever. That doesn't mean that it is being delivered to you with no undiscovered bugs. Things that have "broken" my "sold to the public" software over the years are mostly related to new versions of windows and new versions of Office. When the software was sold, the users were told what version of Windows and what version of Office we would support. So far, we've gotten through four versions of Office and three of windows without me having to make any updates which doesn't mean that MS isn't going to break it tomorrow. We've had a few issues though but the biggest one was last year when they "upgraded" to a cloud server last year and didn't bother to run it by me beforehand. That was a mess but I finally convinced the IT people that they had to listen to me when I told them how the app needed to be distributed. The software hasn't been updated in 10 years. I've offered updates but the clients just like the way it works. it does what they need it to do and they are still very happy with it
Do you have the sourcecode? If you don't have the sourcecode, you won't have any way of actually fixing any errors that are not related to the "environment" and even then, you might not be able to resolve the problem.
Hopefully, you have documentation - both user and developer.
If you are reliant on the developer, you need a service level agreement which defines the response time to fix bugs and an annual contract. Bugs range from annoying to work stoppage. The more important the app is to your day to day operation, the higher level of support you need.