From my details for a late 2009 build
Ibiza 1.4 16V SC, that fuse is 38, a 30Amp fuse and it is for the cabin fan motor.
Personally, I'd not expect that heating up and melting/burning to be due to a higher than normal fan motor current - but it might be, so the fan motor running current needs checking in case the fan bearings are causing the motor to drag and so draw more current, or the motor itself is electrically faulty. I'd think that the true reason for that heating up would be a poor/bad contact at that fuse area, though maybe exaggerated by a higher than normal fan motor running current.
Bottom line, that fan motor wiring should be able to handle currents up to 30 amps if it is fused at 30Amps, though maybe this is just the car industry's way to allow that supply line to initially handle a highish current at start up and the current should drop off as the motor fan reaches its normal running speed - so if the cable is rated at up to that current, then the fuse contacts should also be rated to handle that. It is the highest of any fuse fitted to that style of fuse board, by the heated rear window also uses that value of fuse in position 45.
For your info:- Top row of small fuses are, left to right 1 > 8, middle row of small fuses are, left to right 9 > 16, bottom row of small fuses are, left to right 17 > 24.
Top row of larger fuses are, left to right 25 > 41, bottom row of larger fuses are, left to right 42 > 58.
Edit:- if you look closely at your first picture, you can clearly see that the metal element of the 30Amp fuse is still intact which is what you would expect if the fuse<>contacts were not making good contact and so volts were being dropped across the fuse leg<>fuse board contact - and that is where the extra heat was being generated leading to the surrounding plastic melting as well as the fuse body housing vanishing.
Another Edit:- a tip for everyone, check all fuses annually to make sure that this should not happen, ie push/press them all to make sure that they are still fully engaged in their contacts/holder.