What causes power losses in transmission lines?
I assume that skin effect can be ignored in this case, as the frequency is relatively low (50Hz) and the diameter of the cables is not large.


Actually, the skin effect on aluminium conductors in 50Hz transmission lines cannot be neglected. It's effective for a layer of more than about 10mm. This is easily reached as conductor ropes are typically up to 40mm in diameter.



You have already found the immediate reason for losses, but I doubt that was asked for.

A general view has to take account of losses related to the structure of the power transmission system as a whole.


Your notion of transformers accounts to this.
Then there's dielectric losses on isolation material, which is much worse in cables than overhead lines, and worse at higher voltages and frequencies than lower. DC brings them to nearly zero.
And finally there is losses because of imperfect phase-shift compensation. As such devices are costly, they aren't used in the medium and low voltage range. Here some power is pumped from the substation to the consumers back and forth without doing any work.