I continue working with the issue. Here on support, I found a solved issue concerning embedding full URLs (https://www……) So, I have changed all insert page-references to full URLs and have asked Google Search Console to re-index these pages. I will keep you posted on the progress.
Also, Google Search Console wrongly stated that some URLs were not in the sitemap, even though they were. So, I resubmitted the sitemap. Of course, this has nothing to to with the Insert Pages plugin – but might set someone reading this on track.
Cheers, Norchris