The problem is as stated on the title.

After debug, it appears this is fixed by the [following commit|https://github.com/jenkinsci/file-operations-plugin/commit/da1c222a1f59368624ac1f24494d893127646ab2]: Replace string comparison using '=='

There has been no release since the fix, 15 months ago.

For now, we have to use this workaround, which, I assume, relies on downloading the file from the master before serializing the content to the slave:

writeFile file: 'myfile', text: 'url'.toURL().text