I have the same error and ran php flarum info
and I got
Flarum core 0.1.0-beta.8.1
PHP version: 7.1.30-1+ubuntu16.04.1+deb.sury.org+1
Loaded extensions: Core, date, libxml, openssl, pcre, zlib, filter, hash, pcntl, Reflection, SPL, session, standard, mysqlnd, PDO, xml, calendar, ctype, curl, dom, mbstring, fileinfo, ftp, gd, gettext, iconv, json, exif, mysqli, pdo_mysql, Phar, posix, readline, shmop, SimpleXML, sockets, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xmlreader, xmlwriter, xsl, Zend OPcache
+----------------------+-----------------+--------+
| Flarum Extensions | | |
+----------------------+-----------------+--------+
| ID | Version | Commit |
+----------------------+-----------------+--------+
| flarum-approval | v0.1.0-beta.8 | |
| flarum-bbcode | v0.1.0-beta.8 | |
| flarum-emoji | v0.1.0-beta.8 | |
| flarum-lang-english | v0.1.0-beta.8 | |
| flarum-flags | v0.1.0-beta.8.1 | |
| flarum-likes | v0.1.0-beta.8.1 | |
| flarum-lock | v0.1.0-beta.8 | |
| flarum-markdown | v0.1.0-beta.8 | |
| flarum-mentions | v0.1.0-beta.8.1 | |
| flarum-statistics | v0.1.0-beta.8 | |
| flarum-sticky | v0.1.0-beta.8 | |
| flarum-subscriptions | v0.1.0-beta.8 | |
| flarum-suspend | v0.1.0-beta.8 | |
| flarum-tags | v0.1.0-beta.8.2 | |
+----------------------+-----------------+--------+
Base URL: https://forum.rflintstone.xyz
Installation path: /home/admin/web/forum.rflintstone.xyz/public_html
Debug mode: ON