Moving a Joomla site to a new hosting and now getting missing dependency errors

First, some background information…

I am a medical doctor, not a professional webdev, and I am not very experienced with Joomla. I have only used WordPress in the last few years for some random content.
That said, I have recently been “asked” to take care of a website for a researcher group (I don’t know how else to describe it).

The website was previously administered by a webdev working in another country, who hosted the website on a private server. He told me all the requirements and we managed to find a hosting service which would support the previously built website (mostly concerning php and mySQL).

He sent me the backup of the website and db, we registered the domain transfer, and once everything was done we were hoping everything would work. Of course, we were dead wrong.

I have fixed the configuration.php file, but even after moving everything this message shows up:

Warning: require(/web/htdocs/website_url/home/libraries/vendor/composer/../joomla/string/src/phputf8/utf8.php): failed to open stream: No such file or directory in /web/htdocs/website_url/home/libraries/vendor/composer/autoload_real.php on line 66

Warning: require(/web/htdocs/website_url/home/libraries/vendor/composer/../joomla/string/src/phputf8/utf8.php): failed to open stream: No such file or directory in /web/htdocs/website_url/home/libraries/vendor/composer/autoload_real.php on line 66

Fatal error: require(): Failed opening required ‘/web/htdocs/website_url/home/libraries/vendor/composer/../joomla/string/src/phputf8/utf8.php’ (include_path=’.:/php7.2/lib/php’) in /web/htdocs/website_url/home/libraries/vendor/composer/autoload_real.php on line 66

I have no idea how to solve this issue and it’s really getting on my nerves. The webmaster is very supportive and is (hopefully) trying to find a solution as well, but so far he hasn’t been able to solve this issue either.

Is there anybody willing to help me in this situation? 🙁