Can not import colors from other *.sla file

[founder]

Just wanted to work on the German version of my book

Calculation ERROR.sla -> English version
Calculation ERROR german.sla -> German version

Since my last edit, I added some colors in the English version.
So as I did last time

Menu -> Edit -> Colors -> Import

In the file selection box, I select Calculation ERROR.sla
but I get an error message

The file does not contain colors that can be imported.....

I closed Scribus and restarted. Same problem.
Why did this work last month with the same 1.4.0 version?