Is it permissible to pray behind an imam who makes لحن (grammatical or pronunciation errors) in the Quran, and what should I do if I am next to a mosque whose imam makes لحن in the prayer?

We advise you not to dwell on these intrusive thoughts. You should not abandon prayer in the mosque merely because you suspect the Imam makes errors in recitation. The default assumption is that the Imam performs his prayer well. Furthermore, an Imam's linguistic error (laḥn) in the Friday sermon is not an excuse to miss it. You are not required to repeat every prayer in which the Imam makes an error, because an error can sometimes invalidate the prayer and sometimes not, as has been detailed in previous fatwas.