php - TYPO3 migration from one server to another. Now can't log into backend -

i have taken on management of old typo3 instance 3.8.x. had migrate old server onto new one. old server php 4.x new server 5.x , assume mysql version latest too.

migration went surprisingly well, 1 image displaying differently.

now, problem can't log backend.

i use correct username , password, javascript , cookies enabled. when click log in, takes long time page loading icon, , goes error 500 - internal server error.

i've flushed caches, truncated old , fe user sessions (not there fe users)...

the thing can think of new web hosting settings preventing me logging backend? error 500 go by, i'm bit stuck...

any suggestions appreciated,

thanks in advance.