@Julian124, thank you so much for taking the time to share your experiences with me.
I exported with 1P version 7 and then viewed the *.1pif file in BBEdit. But I did not notice any weird looking entry.
Nevertheless you helped me, because you gave me a new idea which finally led to success.
Here's what I did (I describe it a bit more detailed so that other users who have the same problem can try if this way works for them)
In 1Password I did not export all elements, but selected 200 items in the vault I wanted to export.
Then I exported them via File -> Export -> Selected Elements
I named the export file "Export_part_1.1pif".
In Enpass I selected the folder "Export_part_1.1pif" via File -> Import > 1Password -> PIF at "Folder" and clicked on "Continue".
Enpass then showed that 200 items can be imported and I clicked "Continue".
After a short time I got the message that 200 elements were successfully imported.
I checked a large number of imported entries randomly to see if the import including the attachments was correct and I was happy to find out that the data including attachments was imported correctly.
Then I repeated steps 1 to 6 (export as "Export_part_2.1pif", "Export_part_3.1pif" etc.) and each of the imports worked without any problems.
In the end I again checked the imported data randomly and it looks like all elements and attachments were imported completely and correctly to Enpass.
Now I can go on and get familiar with Enpass and also try to establish synchronisation of the vault.