@paul.buerkner already solved the problem with the intercept, but before that I was thinking about this
For poisson(link=log) they can be too wide, and compared to your data they seem to be too wide. If I read your model right, the intended model has four additive terms for log intensity with each having a prior N(0,1), and thus the prior for the log intensity would be N(0,4^2), which corresponds to assuming that there is 5% probability that the intensity is larger than 720. This may or may not be against your prior information, but at least it seems to be 36 times larger than the largest count in your data. See also @anon75146577’s blog post