I wouldn't think it would have any effect on SEO. If the issue only arises when someone is logged in as Admin, there's no reason the SE's would be spidering those pages since their bots can't login to your site. Also, for what it's worth, there's always a trade off when using content management systems when it comes to validation - particularly if you use some 3rd party modules. Also, while validation does help and should be adhered to as much as possible, the only people I've seen preach about validation and SEO are the "SEO Experts" that try to sell their snake oil to my clients. At the end of the day, it all comes down to well written, keyword and key phrase rich, textual content. A couple alt tags missing from the admin panel will likely have 0 effect on your SE rankings and most likely will be a waste of your time trying to fix compared to the time you could spend improving the site in other ways (adding fresh content, finding ways to engage your existing user base, etc.)
Good Luck!
Some sources:
"...and the basic conclusion is that although W3C is incredibly important for browser compatibility and overall site usability, it does not have a direct impact on Google rankings, especially since Google’s pages themselves do not validate." (
Link)
...and a more recent
posting by Yoast. Quick summary: Definitely fix critical errors (unclosed / invalid tags, anything that would cause a browser to 'break' when viewing the page, etc) and try to get the rest, but don't sweat the small stuff.