antcarrier

It doesn't have anything to do with Omnisphere. I don't have any Spectrasonics products, and this is a huge problem for me, often requiring a restart after only minutes of use.

I'm not using the sequencer either. I just tried to resume work on my latest project...

I have the exact same thing.

I also get this. The only workaround seems to be 1. restarting Logic or 2. recording audio as you go, every time you want the playhead to move (not practical).

I am experiencing this a very similar problem in a major project that I am working on, ever since updating to Logic 10.5 (OS 10.14.6). It typically works for a few minutes, then I have to quit/reopen logic for it to start playing again. Alternatively, if I record audio, it plays back from the playhead just fine. If I simply push play, it will only play for split second, then stop again. This is very annoying as I don't want to be recording audio all the time just to be able to hear what I am doing. Unlike the OP, I cannot play the project from the beginning - it's either record or nothing.