To be honest.... it never crossed my mind that that could corrode also... The other electronics (receiver, main servos, BEC) seem not to be affected, and I also never really heard anyone here about that being an issue. I was fairly surprised that the ailleron servos gave out. I expected them to fail due to being cheap, not due to being corroded, but both servo's (same type) failed in idential manner, hence the conclusion. The servo extension leads seem to be a bit affected as well, probably good for now but I will replace them next year.

I have no way of re-shrinking the driver, so I won't open it, but it is good that you pointed me to it: Next year I'll bring a driver, shrink tubing and some laquer.

But humidity is at times extremely high here, and I have flown a few times when I ended the flight due to rain threatening to start (Noticable by the plane passing through the lower cloud-base), so it is very possible that the ailleron servo's were exposed to water mist and that did them in. After all, the corrosion on the bolts and such was also mainly external. The leg of the tail wheel was even pretty heavily corroded.
IF the old driver turns out to be corroded, I'll replace it, but if it still works, I'll cut off the shrink tube, coat the electronics, and reshrink it.