Hi Jose, there is definitely something wrong. I have gone to a second site updated to 3.1.7 (this site does not have k2, has SP page builder). And I am having the same problems. After upload, I open Page builder and just try to save the page. Off it comes "forbidden access".
I'm not able to reproduce this issue either; can you please send me administrative credentials to any of your sites? There must be something firing the sql injection rule...