After opening the Duplicati main page, this error message popped up:
After setting the Duplicati main interface to English, the error message stays in Dutch.
Translating back to English (Google) results in “The author of a copy method can not have this that is null.”
Note that “this” and “null” seem to be placeholders and are not translated.
As I’m the translator to Dutch, I supposed I should be able to find this string in Transifex, but I can’t find it. I guess I’m doing something wrong, or enter an incorrect search term, otherwise some weird automatic translation is taking place somewhere.
@kenkendk’s answer explains it: My Dutch .NET framework on my Dutch OS running Duplicati presents me a Dutch error message. I assumed it was generated by Duplicati, but that’s not true. Case closed!