Thanks a lot for your replies! So far, I have found two eggs of (presumably) C. nupta: one on Salix alba and one on Populus x canadensis. On the exact same places as where you would expect C. fraxini eggs. Like you both have described, the eggs of C. nupta seem to have a finer structure. To me the ridges seem more apparent. Maybe this is because the ridges in the chorion of C. nupta eggs are actually folds, contrary to the ridges on the eggs of C. fraxini. Like described by Kolesnichenko & Sidorov (2019; 2021).
In the Netherlands, fraxini and nupta are the only Catocala species on Populus. As far as I know, C. electa went extinct a long time ago. Nupta is far more common than the very rare fraxini. Hence, when I posted my egg findings on de Dutch observation.org, a couple of admins started asking me questions. How do I know with absoluty certainty that these are the eggs of fraxini? A right question, when taking into account at the limited available comparitive material on the internet or in literature. When I look up 'Catocala nupta eggs' on Google, some eggs -like shown here- have striking similarities with the fraxini eggs I found.
For now I stand by my opinion that I really did find live eggs of Catocala fraxini . I'm glad that you think the same!
Best regards, Silvio