My thought is to repeat the process, delete the dictionaries from the device, add and download the dictionaries on the device, and replace the files in the dict folder.
I don't know exactly why it happened, but it might caused from unfinished sync, you can cancel the automatic sync in the setting.
EDIT:
here (Kobo Devices) is a guide how to add a custom dictionary, in the folder .kobo/dict/.you need to replace the the file "dicthtml.zip" with the file "dicthtml.zip" that you need extract from
Websters1913.zip.
Don't rename the file dicthtml.zip to "dict.html" or any other name otherwise the device will not be able to use is.