Friend wants me to keep server and software running basically for free

I wrote some software 6 years ago and had a company with a friend. The software ran for years with barely any issues, requiring almost zero maintenance.

The software and company are still alive today…barely. Covid killed most of the business, and now the software is limping along on a very old Ubuntu server that is way out of support.

The software mysteriously keeps crashing every few days, and I have to constantly log into the server and reboot. I spent many hours investigating the root of the problem and was unable to find a fix. I am a senior developer with 23 years of experience, and I am completely stumped…I just cannot figure out why the software/server are failing.

I sent my friend an invoice for 5 hours of work, and he refuses to pay because the problem is not fixed. I told him that this is an extremely difficult problem, it is an iterative process to solve, and I have to try different things over time to see what works. I also told him, before trying to fix the problem, that things are basically held together by duct tape at this point, and he should just terminate the business (which literally breaks even and makes no money).

I even offered to reduce my rate by 20%, and he still refuses. Basically I am working for free until I can fix the problem.

Am I in the wrong for sending him a bill when the problem is not solved? What would you all do?