I also had similar issues on iOS when I migrated to Baikal. Every other device was working fine.
Double check that your certificates are valid. I don't remember the exact details but Apple made a few changes somewhat recently in regards of what kind of certs are accepted on their devices.
If you're using self signed certs you have to install them manually as well, don't forget that.
Was looking for the same for ages. Every option I found was either some bloated docker solution or a dead project.
Gave up and just created a repo named "Todo" on my git server. It's not really designed to be used for this kind of stuff but git and a web ui works perfectly fine. Just open a new issue. Give it a title and a brief description, a due date and some tags. Then you can create more subtasks, add comments to each, references etc. Even got my non-technical wife to use it.
With some customization you could probably get exactly what you want.
Edit: just realized you probably wanted something more simple. Set up a caldav server and sync the tasks using your preferred to-do app.