What to look for when migrating a site to opencart?

Question

opencart

Hello.
Moved the site to Opencart from Shared to VPS. Everything works at first glance, but there are problems with sessions, as I understand it.
For example, it is thrown out of the Personal Account when entering the basket.
The log shows a lot of things like this:

2019-04-11 13:40:31 - PHP Warning:  mb_substr_count(): Unknown encoding "8334" in /var/www/a9496/data/www/site/system/library/template/Twig/Compiler.php on line 220

And here is this:

2019-04-11 15:38:25 - PHP Warning:  unlink(/var/www/a9496/data/www/storage/cache/cache.catalog.language.1555000693) [<a href='0function.unlink0'>function.unlink0</a>]: No such file or directory in /var/www/a9496/data/www/site/system/library/cache/file.php on line 68

The rights to directories 755, to CACHE and SESSION set 777. I cleaned the CACHE folder. TWIG - I don't know how to clean it, unless I buy a module. But I did it according to this instruction: https://webocreation.com/blog/opencart-twig-cache-...
What could be the problem if the site was transferred by AS IS? Management system - ISP, Apache, PHP 5.6