I can’t reproduce your issue. It seems your txt file associations are not correct.
Can you try associating another file extension (such as .log) from the EmEditor tools menu, Customize, File tab, click Associate with EmEditor button, and then add .log to the list. Then please try opening from Explorer. If this works and if .txt doesn’t work, you might want to remove .txt from the association list, and add again. You can also try uninstalling EmEditor completely, and the try installing EmEditor again.
I hope this helps.
P.S. The “Customize Toolbar” dialog is a part of the OS.