Cannot upload modified Stylesheet or Theme Functions

Hello,

When I try uploading a modified style.css or functions.php from the editor, I get an “Internal Server Error” which I haven’t been able to track down the reason for.

It’s only effecting these two templates as I went through every other template in the list, added a space to the comment file to modify it and they upload ok i.e.

404 Template
Archives
Attachment Template
.
.
editor-style-rtl.css

Any suggestions on how to troubleshoot & resolve this are very welcome.

Thanks!