My WHMCS worked well before it updated to 7.5.1, from last Saturday it became very unstable.
At first, I received the Cron Daemon email last Saturday which said Cron ran failed. The detail is '...PDOException SQLSTATE[HY000]: General error:1205 Lock wait timeout exceeded;try restarting transaction...'
So I tried to open my whmcs system https://www.hostfans.com/billing, it ran slowly. When I tried to open the admin area to login to the system https://www.hostfans.com/billing/realtowebjerry , it just waiting... no respond. I tried to use SSH to run the Cron job, no respond too.
I went to the Cpanel, the WHMCS Database can't be open. But other Database ran fast.
I contacted the Hosting Support Team two times, they tried to help me. Everytime they said '...there were some long-running queries of your database. We killed those queries and your site is now loading for me....'
After they killed the long-running queries , the https://www.hostfans.com/billing opened fast for a while , but when I tried to login to the admin area https://www.hostfans.com/realtowebjerry, the WHMCS system became very slowly and no responding again! then the .../billing can't be open too.
Inmotionhosting provided the hosting space, PHP 7.0, whmcs 7.5.1