Even though the problem is not with MyEclipse, I thought I’d post a follow-up for this issue as I have found another cause, just in case somebody else is searching for a solution.
I installed the VI editor for Eclipse (version 1.10.6 from http://satokar.com/viplugin/index.php). This caused the same errors. The editor generally worked well, but I did recieve the Save Failed:null error when trying to save my struts-config.xml files. I had no problems with any other files (even other xml files).
Closing the file (without saving), turning off the editor, reopening the file and making the changes again (without the vi editor) seems to allow saving of files.