That's great that you figured it out. Would have been nice if you could have posted how you resolved it. Then when others like me are searching the forums for solutions, we won't be led to empty worthless threads like this.
For anyone else having this problem, I resolved it by disabling the "code mirror" plugin.
From documentation / FAQs:
If you have trouble saving your changes, try to disable the "code mirror" plugin via the joomla plugins manager and check that the folder templates/YOUR_TEMPLATE/html/com_hikashop exists and is writable.