Journals not loading

Description of issue or problem I’m having:
I have been using OJS to manage our journals for over 1 year. All of a sudden, none of the journals is loading anymore. We have not made any changes to the journal. Please help me recommend way out.

Steps I took leading up to the issue:

I am helpless about what to do to resolve the issue.

Application Version - e.g., OJS 3.1.2:

3.1.2

Additional information, such as screenshots and error log messages if applicable:

Here are the error logs:

#0 /home/orapuhor/public_html/ojs/ojs-3.1.2-4/lib/pkp/lib/vendor/symfony/polyfill-mbstring/bootstrap.php(38): Symfony\Polyfill\Mbstring\Mbstring::mb_substr(‘Ayoade’, 0, 1, ‘UTF-8’)
#1 /home/orapuhor/public_html/ojs/ojs-3.1.2-4/plugins/generic/citationStyleLanguage/lib/vendor/seboettg/citeproc-php/src/Seboettg/CiteProc/Util/StringHelper.php(159): mb_substr(‘Ayoade’, 0, 1, ‘UTF-8’)
#2 /home/orapuhor/public_html/ojs/ojs-3.1.2-4/plugins/generic/citationStyleLanguage/lib/vendor/seboettg/citeproc-php/src/Seboettg/CiteProc/Rendering/Name/Name.php(181): Seboettg\CiteProc\Util\StringHelper::initializeBySpaceOrHyphen(‘Ayoade Stephen’, '. ')
#3 /home/orapuhor/public_html/ojs/ojs-3.1.2-4/plugins/generic/citationStyleLanguage/lib/vendor/seboettg/citeproc-php/src/Seboettg/CiteProc/Rendering/N in /home/orapuhor/public_html/ojs/ojs-3.1.2-4/plugins/generic/citationStyleLanguage/lib/vendor/symfony/polyfill-mbstring/Mbstring.php on line 597