OK - it seems Gallery has not been updated in a long time and is not compatible with 2.5.8??? The controls to delete / order images have disappeared... Just been forced to move from 2.1.10 to 2.5.8 due to a forced server upgrade to PHP7 which 2.1.10 does not work on... Now the gallery system in this site is a total mess... Anyone any ideas on a resolution????
It looks like Gallery requires php-mbstring Enabled
has the following to say about php-mbstring
mbstring is a non-default extension. This means it is not enabled by default. You must explicitly enable the module with the configure option.
So maybe your new PHP7 install needs to have php-mbstring enabled?