Bringing old install back to life

Hello,
I have been asked to rescue an old install to make some content updates.
I am a newbie with Grav so I am trying to figure out the best way to tackle this.
The website is here: https://www.ghostraum.com/ Access to admin is broken. That's the address I have been given by former admin : http://www.ghostraum.com/panel/users
I have the right logins but it leads to error 500 page.
I am wondering if I should try and upgrade this current site, somehow get access to admin and then change the content OR f I should install a fresh vanilla site and try to migrate the content from old site.
What would be the least painful solution ?
Thanks in advance for your help.
Best regards
Guillaume

Oh ! No wonder I was so amazed at how site architectures were so different! haha
OK Apologies