Hi,
The 501 error can be due to a limitation of the POST data or a security mod.
Because the email edition allow you to edit 3 files in the same time ; that the content includes HTML content ; it is possible that the POST length is reach or that something reject the content.
At this moment, you can try a little thing.
Edit an email and empty two text areas (the "preload" and the "text" the first time ; the "html" and the "preload" for a second test).
Thanks to these two tests we will be able to know a little more about your issue.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.