see ?stan and option seed. Try runinng stan(...., seed=1337) with different seed values.
see ?stan and option init. The default is to randomly generate initial values between -2 and 2 on the unconstrained support. If you transform -2 and 2 from unconstrained to constrained space, are these initial values sensible?
If you are unlucky initial values are all 2, and if n=1 and x=1, then
log(n) + x*2 + 2 + exp(2)*2 + epx(2)*2*4
is about 79 which when exponentiated to Poisson parameter is about 2e34, which is a lot. You probably need to initialize log(a_std) and log(b_std) with much smaller range than [-2, 2] or scale them in Stan code by dividing them with a big number. For example, try dividing ‘a’ and ‘b’ by 100. Then maximum Poisson parameter value (with n=1 and x=1) is about 311, which should work fine.
This is something we should add in wiki / manual (if it’s not already there). Sorry I didn’t realize earlier this is the likely problem in the other thread,