There is an error in the dictionary file, so you will need to delete it from the database, then download it again.
To do that, close eM Client and open Windows Explorer. Go to C:\Users\yourusername\AppData\Roaming\eM Client\Dictionaries. You may have to enable hidden items in Explorer to get there. Delete the two nl-NL.* files. Then restart eM Client and go to Menu > Settings > Spell-checker and download the dictionary again.
Seems like you did not close eM Client before deleting the dictionaries, so they app still thinks they are there. Restarting eM Client after deleting the dictionaries (or rebooting as @skybat suggests) will allow you to do that.
I took the files from another eM Client (which has worked well) and pasted them into the appropriate directory. Result was the same. Bad.
Now I’m going to uninstall and reinstall eM Client. See if it is resolved then.
Ik denk dat je dus Nederlandstalig bent. Ik heb het toen opgelost door de woordenboeken van een oudere versie van emclient te installeren. (Gelukkig ik heb 3 computers). De grootte daarvan was 1881.063 Bytes. En dat werkte beter (nog niet perfect.)
Ja, ik ben Nederlands. Heb ze meerdere keren gedownload zonder succes, maar de laatste keer werkte het wel.
Not sure what changed, but the last attemopt worked.