in a domain gpo startup script, what user does the script run as?
computername$
(i.e. the domain computer account aka LocalSystem)
Startup scripts begin executing before any user has logged on, so they must run in the context of a builtin security principal. non-domain-members or Local GP startup scripts will run as LocalSystem. In a domain-joined scenario, LocalSystem is equivalent
to the computer account.
If your script is needing to access network resources (e.g. a file share or a website), those network resources must grant access to computername$, or, Domain Computers, or, Authenticated Users, or some equivalent security group which represents domain-member-computers.
If your script is failing, some logging within the script might help you identify where and why it's failing. (e.g. insufficient permissions)