The Shopware error message "core.ERROR: exception 'Doctrine\DBAL\Types\ConversionException' with message 'Could not convert database value "s:0:""" to Doctrine Type object'" or a similar error message may indicate invalid values in the "s_core_config_elements" table. If a terminating semicolon is missing after an entry in the "value" column, Shopware (under Linux) cannot process it (whereas Windows can).
The solution is to manually add the semicolon to the affected rows or do this automatically using an SQL query.
UPDATE s_core_config_elements SET value = CONCAT(value, ‚;‘) WHERE value NOT LIKE „%;“;
Everything will then work again without any problems. Tested with Shopware 4.2.3.
