I doubt if the problem was caused by the cover. To many people are using sleep covers without reporting a problem like this.
What probably has happened is that the font engine crashed. Restarting the device restarted all the software and this worked again. Why did it happen? I don't know. It could be a book related problem, a memory leak or something else. If you look in the .kobo directory, there might be a log file from when this happened. The file name will be "stack_nn.log" where "nn" are numbers. There can be up to ten of these. These, with a description of what you were doing when they were produces and possibly the book, can help Kobo developers to work out what went wrong and fix it.
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