Cron Error and Admin login make the site down

[jerry.gai]

       My WHMCS worked well before it updated to 7.5.1, from last Saturday it became very unstable.

1. 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...'
2. 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.
3. I went to the Cpanel, the WHMCS Database can't be open. But other Database ran fast.
4. 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....'
5. 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. 
6. Inmotionhosting provided the hosting space, PHP 7.0, whmcs 7.5.1 

[jerry.gai]

I found a interesting thing. When I changed the php version on the server, there will be a ionCube Error show, after I use the  loader-wizard.php to install the correct version to server, the ...billling and the .../billling/admin opened fast. but when I tried to login to the manager area, the system became slowly and no responding again! 

[jerry.gai]

The reset password function is good to use. I can reset my administrator's password. But I still can't login to the manage area! It seems  the dologin.php have some problem?

When will you respond me? I have waiting to enter my Admin Area for five days!