For me personal it would be guessing here (this case) e107 on Ubuntu;nginx works.
But don't like php 7.4. Furthermore Cloudflare...??
As i do not use this kind of setup, i would more think indeed of caching or session problems or, since v1 and v 2.3 differ quite a lot. Cloudflare ip 'handling'. (same origin and alike)... But in case of cookie cache, even better was ALL cache related (even settings in admin on unused; not most nice one). Or security level (5) maybe to hard already (nahhh best not to tinker). And of course from dev to live.. is the site url (domain) properly set in site,and cloudflare... Though to tell here because everybody uses different things,, Although it is described as upgrading went ok, maybe a present v1 plugin has issues anyhow......