You can find more information about the problem, cause and repro in the following thread: http://www.telerik.com/community/forums/spell-check-hander-error-the-length-of-the-string-exceeds-the-value-set-on-the-maxjsonlength-property
The issue is preventing the users from adding a word to RadSpell custom dictionary. A workaround is attached. You can apply the DialogScriptFile property to all RadSpell instances through an ASP Theme. The bug will be fixed in Q1 2015 SP1, so as soon as you upgrade to this release (or a later one), you should remove the workaround.
RadSpell's dialog should have proper tabIndex order to provide keyboard navigation. You can find attached a sample that lets you provide the desired order. When the buttons are disabled the browser should not focus them (i.e., the disabled attribute should prevent this even if tabIndexes are not modified). Related to this improvement request: http://feedback.telerik.com/Project/108/Feedback/Details/126935