common causes.
1. Temporary Files Full
When the Internet Explorer disk cache becomes full it may stop the View
Source function from working. To resolve, open Internet Explorer and click
"Tools" > "Internet Options" > "Delete Files".
2. Cookie Folder Location
This issue can also occur if your Cookies folder is located on an NTFS
partition and you do not have at least Change permissions for the Cookies
folder. To resolve, make sure your account has sufficient permissions on the
Cookies folder.
3. Missing Notepad.exe
The View Source function uses Notepad to display the HTML, if "notepad.exe"
is missing from the Windows directory then it will not work. To resolve,
ensure a copy of Notepad.exe is in the Windows directory.
4. Incomplete Page Download
If you press the ESC key before the page has completed loading then the
"View Source" menu item may not be displayed.
5. View Source Restriction
The View Source function may have been restricted by your Administrator.
6. Invalid Temporary Directory
You may also receive this error if the path specified for the TMP
environment variable is invalid. To resolve, make sure that the TEMP
directory is a valid directory and that there is sufficient space on the
drive.
7. Invalid View Source Program
If you have specified an alternate program to view the source code.
8. Right-click disabled by Web Page
Some web pages may implement a restriction to stop visitors from
right-clicking on the web page. The source may still be viewed by clicking
the main "View" menu, then "Source".