Database error when search articles as editor

Dear All;
i experienced slow search when i login as editor since one week; i get the following error “DB Error: MySQL server has gone away” two to three times within the last week when i try to search for an article.
today the error is persistent; and any search when i logged in as an editor returns this error “DB Error: MySQL server has gone away”. also if i try to click the archive articles as an editor i get the same error; if i try to click the "in editing: articles i get the error some times.
any idea?

note: we did upgrade from 2.3.8 to 2.4.8 since 21 September 2017. we get at the time of upgrade the following error “• A database error has occurred: Table ‘moanassar1.article_event_log’ doesn’t exist”. but it dose not affect any thing except when searching using the general search at the site level for visitors we get “404 Not Found”; bu other than that every thing was going ok since 21 September 2017.

This seems to indicate that there is a problem with your SQL database. The error message can have a number of causes, but if your site level search is not working, it could be necessary to fix the error that happened when upgrading. Do you have a backup copy of your old OJS 2.3.8? Probably, if you keep using this broken database and upgrade it further (current OJS version is 3.1.x), it is likely that the problem is not going to disappear but to possibly grow.