Cannot use more than one core with pymc3

Question

Every time I use more than one core on my pymc3, I get the following error -

RuntimeError: Chain 0 failed.

Here's my code -

import pymc3 as pm
import numpy as np
import matplotlib.pyplot as plt
obs_y = np.random.normal(0.5, 0.35, 2000)
with pm.Model() as exercise1:
    
    mu = pm.Normal('mu', mu = 0.0, sd = .05)
    stdev = pm.HalfNormal('stdev', sd=.05)
    
    y = pm.Normal('y', mu = mu, sd = stdev, observed = obs_y)
    
    trace = pm.sample(1000, cores=2)
    pm.traceplot(trace)
    plt.show()

This problem doesn't occur when I use the following code -

trace = pm.sample(1000, cores=1)

Here are my library versions. My objective was to run my sampling using GPU, therefore I installed pygpu, which might have caused the issue.

Theano-PyMC          1.1.2
pygpu                0.7.6                                                                                              
pymc3                3.11.5