Just hit this same issue the next turn after negotiating a peace.
Edit!
After simply skipping a turn with one trader the problem seemed to resolve. Although I did also gain a world wonder this turn. But at least I can now trade again.
Edit again!
That was shortlived; seems like I have to be picky about which city the trader is in. At the moment I can only trade in the city I just gained via conquest.
Edit more!
Oh, I seem to have lost trade route slots, I now only have 6 instead of 8 I had before. So probably lost when switching government type, but changes had effect so much after that it took a while to figure it out.