Hi Alba,
Thanks for the screenshots — they pinpoint the cause, and it isn't SP Page Builder or Joomla.
Behind the generic 500 page is this database error:
User 'o13*******' has exceeded the 'max_user_connections' resource (current value: 4)
Your MySQL user is limited to 4 simultaneous connections. The front-end editor loads page data, addons and media through several parallel requests, each needing its own connection, so it goes over that limit almost every time — while normal page visits stay under it. The "error in your SQL syntax… near ''" message you saw is a side effect of the same dropped connection.
Two things to do:
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Lower what your site needs. In System → Global Configuration → System tab, change Session Handler from Database to Filesystem, and set Cache to Conservative Caching. Also keep just one tab open on the site while editing, and turn Debug back off once testing is done.
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Ask your hosting provider to raise the limit. Four concurrent connections is very low for a CMS — most shared hosting plans allow 20 or more. You may ask them:
My Joomla site returns HTTP 500 errors and MySQL reports "User '…' has exceeded the 'max_user_connections' resource (current value: 4)". Please raise max_user_connections for this database user, or advise which package provides a higher limit.
Step 1 often gets the editor working again straight away; step 2 is the permanent fix. If anything remains after both, let us know and we'll continue from there.
Best regards,