Hi Ciaran,
I just tested Joomla 5.0 RC, which I installed after updating to Joomla 4.4. The Update to 5.0 RC ended up by this message: Call to undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin().
Looks like an incompatibility, most probably of your template Nina with all the extensions?
Do you have a solution?
Thanks, Werner