It's indeed due to the different capitalization ( c: vs C: ).
You need to change it in the registry where the service configuration is kept: HKLM\SYSTEM\CurrentControlSet\Services. Find your instance and look at the ImagePath key. It'll have either a small c: or capital C:.
It's a bit strange that you receive this message at every restart because once SQL starts it records the "correct" value in sysaltfiles and it shouldn't complain at the next restart. At least that's what I saw while testing.
Hope this helps
Just figured out why it could give you that message more than once. If you have a cluster the nodes may have different values. If they failover/back you'll see the message again. So you need to change all the nodes in your cluster