Solved by changing PC and USB port. 4 of 6 ports caused this error.
Mission Planner 1.2.53 (latest)
OS: Windows XP
All drivers downloaded, installed and checked just before the test. APM moved to COM4.
After uploading different firmwares got this message: "upload succeeded but verify failed", each time with different byte addresses. Tried on two laptops with total of 6 usb ports. Only 2 ports continuosly upload without errors. Others fail.
They fail only in MP upload. Mouse and generic Arduino uploads appear to be flawless.
I think this behavior must be noted in the manual so users will have in mind that they better have more than one computer on hand before working with Ardupilot.