Spellchecker stopped working after update from 4.2.5 to 4.2.7/4.3.4

Component Writer
O/S Windows 7 professional EN
LibreOffice locale NL

All normal settings are so to have spellchecking active and the language Dutch.
But no spelling error is shown/reported althought here are plenty in the document.
F7 simply asks whether checking should be started from he beginning and the the checking has finished. The spellchecking dialog cannot be entered.

On other machines the document works fine, with wavy lines below typing errors.

How to fix this?

Could you please check in
Tools > Extension Manager
if you see there the Dutch dictionary. If yes I am have no answer yet; if no, I would repeat the installation and do it customized to make sure the the Dutch dictionary is selected for installation.

I once also had a problem with missing dictionaries and a customized re-installation solved my problem. (XP machine)

Thank you!
Indeed it is missing. As all machines in our company have a Dutch O/S, we never bothered to look into extensions and updates are without user interface.