>the oxidation of formaldehyde to CO2 is avoided
From what I understand, that's not a problem. In fact, you WANT formaldehyde to be oxidized.
It's formaldehyde that's oxidized to form formic acid--that's rxn drives the reduction of the methyleneimine. That HCOOH is of course further oxidized (but ask yourself the question--what is reduced?) to water and carbon dioxide.
So if you answer that question, maybe it's that further oxidation of formic acid occurs because it's being used to reduce more methyleneimine.hcl. But that's just a guess. I don't know enough about the mechanism of this rxn -- except what's on
https://www.thevespiary.org/rhodium/Rhodium/chemistry/methylamine.html
Dimethylamine production is a problem and results from formaldehyde reacting with methylamine. (how would addition of alcohol supress that? other than to keep the temperature low, which is somethign that could be accomplished by slow heating anyways... my guess would be that perhaps removal of the formic acid helps drive the reduction of methyleneimine, a Le Chatlier principle thing but who knows... and but why not add methanol instead of ethanol? methyl formate would have a lower BP than ethyl formate)
Also, Organikum, are you sure what you got is pure methylamine hydrochloride? If so, I believe 10g of it will dissolve in less than 10ml of water. Check that to make sure it all goes into the water, if not, then you know you've got ammonium chloride as well. Eleusis made a mistake where he thought his yields were awesome when they were filled with mostly ammonium chloride
Abolt, make sure your methylamine hydrochloride was absolutely dry (not a sludge... it should be a rock hard CHUNK in your flask after distilling off the water) to make sure you really got all of the methylamine hydrochloride from the rxn via recrystallizing with methanol.