Re-Installing latest wp

Hi
I migrated the whole website and db to a different server and have major problems since (cannot update theme, plugins… and also automatic wp updates fail). I run multisite version but do not yet have any subsites.

When trying to delete the theme in order to manually reinstall it, it will not let me delete the theme, claiming it did not exist? I really do not know, where to start…???
Another issue I encounter: Plugin “updraftplus” for backups I deleted in order to reinstall. It worked and I even performed a backup – but at present, the plugin is NOT showing in wp backend under the plugin list? It disappeared – but in the ftp it still shows….?

I was thinking of starting by “reinstallling” the whole wp to make sure I run the latest version. Could you have a look at my backend or any log to assess, which way to go? If you suggest to reinstall wp, could you guide me?

I really appreciate your help. Kindly,
Charles