I used the backup function and restored my accounts into 2.4. Problem is solved. Also the language settings are ok now.
Have still issues with the time zone settings. I did not change anything. There is nothing to change in line 50 of the config.php and config.default.php file as asked in both warnings. "date(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected the timezone 'UTC' for now, but please set date.timezone to select your timezone. in bestand: /hsphere/local/home/a986034/debeesnv.com/frontaccounting/config.php op regel50"
My timezone is CET Brussels but the server is situated in the US
Time and date used by frontaccounting is correct.