Can you please describe in more detail what happens when you try to update plugins?
The appearance of my WordPress when i try to update themes etc. is like the attached screenshot
This looks like a server-side error. Are you accessing the backend via https? If not, do that and try again.
Otherwise, I would recommend you to contact the support of your hoster.
I previously installed an https security header plugin, but I can’t recall its name since I removed it some time ago. Do you have any other suggestions considering I genuinely forgot the plugin’s name, :[
The only thing that comes to mind is Really Simple SSL:
If this has left any residue in your hosting, a look at the .htaccess file might be useful. The support of your hoster can also help you with this.