If you visit the thread in the Kobo forum here for firmware 4.8.10956, you will discover this is a problem with the firmware. Kobo changed the font used when you select the "Publisher default" option. This is causing the problems you describe. It particularly affects the Kobo devices with only 256MB of RAM. This includes the Touch and Glo and probably the Aura Edition 2. It will also happen if you choose on of the two Kobo fonts near the bottom of the font list.
For this reason, Kobo have stopped sending the update out to devices. They are working on a fix and I hope they will have this out next week. This fix will probably change the default font use for epubs back to Georgia.
Until then, you can revert to the previous firmware version if you want. But, choosing a different font on the device for the book, closing it and reopening it should fix it.
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