I install OK get admin OK - site fine - develop nicely then... ouch, I am either logged out when I try something in admin or I have logged out and can't get back in. This is locally. But now I am migrating online and - again. I will explore all my variable in the hope that they can be excluded or commented on. NB I have pretty slow internet (when I have it).
1 - My assumption is that login data is held in a session - yes?
Locally, when there is a 'crash' I find that another install using the same password allows me to return to the earlier dev site admin. This is easy enough offline as a new install is quick. Online is slow

What I use is a modded version of 3.15.0 - I don't think my mods would affect admin but briefly:
a) delete themes, leaving blogx - in install.php > 'theme' => 'blogx',
b) blogx is considerably modified by me
c) delete all plugins - replace with my set of 5
d) delete all languages in theme/plugins - except en.json
2 - Will any of a-d affect admin?
Working online:
a) I closed xampp
b) I delete all browser data: cookies & cache
c) ran the online install - with a never used pwd. = fine
d) login = fine
e) logout > login = no go
f) I tried a local install with the online pwd. then tried online = no go
3 - What else am I missing?
I have read quite a few other posts but am sure the uer/pwd is correct.
I hope the above is clear and someone can offer some suggestions.
Thank you.
Kusalo