Issue 1 happened during an update from version 1.7.6.4 to the latest stable one.
After the DB restore, the shop itself run well, but the admin backend run into an error
prestashop FileLocatorFileNotFoundException
solution: download and install the 1.7.6.4 version and copy the src directory from the fresh one, renamed first and deleter afterwards the corrupt one.
Issue 2
Upgrade of modules was not possible: classes/../tools/pear/PEAR.php): failed
solution: removed blockrss module and replaced other corrupted modules which run into the same issues from modules of fresh installation