I had similar issues that came down to the metatag in my custom master. I had set it on content="IE=10," and then users started reporting they could not edit text in Rich Text fields on forms, could not open lists in Excel, and several other problems. Changing ti to IE=9 did not fix all the issues, so I had to set the metatag to IE8. That is the only way to correct ALL the problems that users were experiencing. I think the "real" answer is to upgrade to SharePoint 2013, but that is a major effort, and the client I'm working with is simply not ready to do that right now. Hope this helps.
<meta http-equiv="X-UA-Compatible" content="IE=8"/>
tag (or increment it up above "IE=8"). – Stevangelista Jun 17 '14 at 15:10