Is delivering a lesson or lecture before the 'Isha prayer in Ramadan an innovation (bid'ah)?

There is no objection to delivering lessons in mosques before or after the prayer. The matter of timing should consider what is beneficial and when people are most likely to benefit. This is supported by the actions of the Prophet, peace be upon him. It is authentically reported in Sahih Muslim from Amr ibn Akhtab that the Prophet, peace be upon him, led them in the Fajr prayer, then ascended the pulpit and addressed them until Dhuhr prayer was due. Then he descended and prayed, then he ascended the pulpit and addressed them until Asr prayer was due. Then he descended and led them in prayer, then he ascended the pulpit and addressed them until the sun set. Similarly, Al-Hakim narrated, and authenticated, that Abu Hurairah, may Allah be pleased with him, used to address people before the Friday prayer.