Most server technology runs on Linux. A linux server would allow me to use my existing server infrastructure
I agree - this is a huge impediment to adoption for some people. I too don't want to have to fork out for a windows box and licenses. This is probably the best time tracking system on the market and I think you are blowing by not supporting Linux and (in particular) Mac.
PS Would a windows IoT server on a Raspberry Pi be a suitable host?
Pierrick Puimean-Chieze commented
Totally agree. I was (still is) a huge fan of Manic Time, and want to be able to use the mobile app. But I don't want to pay for windows license for my server. Especially when the only app I would run on it would be Manic time