Recently I had an issue where the OK button was grayed out on the commit dialog. This was confusing. The command line client was working correctly and the repository was fine.
It turned out that I had forgotten about the property tsvn:logminsize which was set to 10.
This property defines the minimum comment size that must be entered before you can commit your changes. This stops changes being committed without any comments.
If I had entered my comments first the OK button would have been enabled.
Hope this saves someone else some time!