Step 1
yum search mbstring
Step 2
yum install php71-php-mbstring.x86_64
Step 3
find / -name "mbstring.so" -print 2>/dev/null
Step 4
ln -s /opt/remi/php71/root/usr/lib64/php/modules/mbstring.so /usr/lib64/php/modules/mbstring.so
It is was just a temporary workaroud.
Later, I have updated the broken PHP installation to 7.2: How do I install PHP 7.2 FPM and CLI for Magento 2 to CentOS 7?