welcome to the notepad++ community, @Werner-Vogelweide
unfortunately the DSpellCheck plugin still has some issues on files with very long lines without line breaks.
the only workarounds possible at the moment are these in this order (you can stop at any point, as soon as your issue is resolved):
go to plugins > plugins admin and update to DSpellCheck 1.4.9 if you are on a version below that.
then restart notepad++ and retry with the same documents.
go to plugins > DSpellCheck and disable spell check documents automatically
then restart notepad++ and retry with the same documents.
go to plugins > plugins admin and remove DSpellCheck completely as a last resort.
note: the DSpellCheck plugin will probably not be installed by default any more, when conducting a fresh install of the next version(s), according to the developer, due to the amount of issue reports. it will remain as an optional install at the plugins manager.