In the paper Einstein's controversy with Drude and the origin of statistical mechanics: A new glimpse from the "Love Letter" by J. Renn, (1997), we read:
The nature of Einstein's objections to Drude's theory [...] has remained just as unknown to us as the character of Drude's response to a letter we know Einstein had written to him around early June 1901.
Renn then notes how some new and relevant information has come to light recently. In particular, a letter from Mileva Maric to Einstein concerning Drude's response to Einstein's objections. Apparently Einstein had forwarded Drude's response to Mileva, so Mileva's return letter to Einstein offers a credible source. From this and other available information, Renn argues that Einstein presented two "factual objections" to Drude's theory.
Renn proposes that Einstein's first objection was to a gap in Boltzmann's gas theory, which Drude had assumed in his own theory and explains why Drude felt the need to defend Boltzmann in (what we know of) his response to Einstein's objections.
Einstein pointed to the achievements of the kinetic theory in the field of gas theory but also to its failure to derive, from mechanics and probability theory, theorems on thermal equilibrium and the second law of thermodynamics for more general systems. [...] From what we know about Drude's response it follows that at least one of Einstein's objections must have been directed at the statistical assumptions introduced by Drude and their justification by a kinetic theory of matter.
Drude appears to have disputed the very existence of the gap which Einstein intended to fill.
According to Renn, Einstein's second objection was related to a problem already noted by others concerning the compatibility between kinetic theory of heat and what was known about the behaviour of matter from other contexts.
Given the lack of direct contemporary evidence concerning Einstein's second objection to Drude, one can only rely on later references to the theory of electrons in metals and its problems in order to identify traces of this objection.
The (PDF) pages 20-22/40 of Renns paper give a technical description of these issues before Renn concludes of the second objection:
It is hence conceivable that these problems also constitute his second "factual" objection to Drude in 1901.