Hello,
Please check the issue on one of the default themes to understand if our theme causes the problem.
Could it be conected to some ajax conflict witk sidebar login?
No, It is hardly ever. Most of the clients configured AJAX with the Login form in the sidebar and it does not influence the orders placing.
We suspect that this is the issue of plugins conflict. If you see the same situation on the default theme, deactivated all the third parties plugins not related to the theme and activate them one by one to discover which one influence.
Best Regards