HTTP ERROR 500 - Some pages on our website have stopped working

Hello, we encountered an issue with our PHP website last week where the website entirely stopped working and we were seeing the following error:
This page isn’t working
app.xxxxxxxxx.co.uk is currently unable to handle this request.
HTTP ERROR 500

We contacted our web host support and they were able to fix this by doing the following:
"The issue was related to the .htaccess rule, that was conflicting with the PHP Tweaks menu"

The website worked absolutely fine after this for another day.
The following day, I tried to login to the admin dashboard for the website and we received the same HTTP ERROR 500. However, all other pages (other than the admin dashboard) is working.


I went back to my web hosts and they identified the following issue:
“It appears that the issue is caused by the error described in this log:
[05-Jul-2021 17:50:04 Europe/London] PHP Parse error: syntax error, unexpected '{', expecting identifier (T_STRING) in /home/xxxxxx/app.xxxxxxxx.co.uk/apps/backend/forms/LoginForm.php on line 5”

So we decided to do a full restoration of the website from a backup when it was working last week. The full backup completed but the error on admin dashboard pages still occur. My developers did another check on the error and found that the error is now leading to this file:
“I have double-check logs, and found out that it refers to the similar error, but in another file:
[05-Jul-2021 20:18:13 Europe/London] PHP Parse error: syntax error, unexpected 'function' (T_FUNCTION), expecting identifier (T_STRING) or \\ (T_NS_SEPARATOR) in /home/ xxxxxx/app.xxxxxxxx.co.uk/vendor/myclabs/deep-copy/src/DeepCopy/deep_copy.php on line 5

When we first moved over to these web hosts (6 months ago), we had the same issue where all pages on the website were working but the admin dashboard showed HTTP ERROR 500.
The web host support was able to fix this by changing the PHP settings on the website via CPANEL.
They are convinced this issue is unrelated but I believe it could be a simple fix such as this as nothing has changed on the website from when it was working last week and we have taken a full backup form that time.
During the last week I have spoken to numerous support representatives from our web hosts team and I believe they may have changed some server settings or PHP extensions which may lead to us being unable to access the admin dashboard pages.

I am not entirely sure though. Can someone please get in touch to see if they’re able to help us fix the issue? Many thanks.