Notice: unserialize(): Error at offset. What is the error (warning)?

Question

PHP

Today I tried to revive the site on Joomla The site on Joomla does not work. How to fix? Somehow I got it to work on OpenServer, but now when installing on the server (php7) - errors occur

Notice: unserialize(): Error at offset 869 of 872 bytes in /home/www/site.ru/libraries/joomla/cache/controller.php on line 205

and so on half a page. And the error disappears as soon as I clear the cache, but then it immediately returns as soon as you start visiting the page a second time. What is notice? Who faced on joomla?
Update Vremo cured by disabling the cache altogether, but, of course, this is not an option