@dc42 @Wyvern guess the problem is solved: I just switched to a switch (ha) anycubic probe sitting right under the nozzle, and here is the result... which at least confirms the fairly good "trueness" of the frame (for which I ran out of any possible options):
0_1550865301922_Annotation 2019-02-22 205053.jpg
Thanks once again for pointing me towards the right direction!! 🙂