Mysql warnings: SELECT vid , duplicate key, etc

Hello,

I successfully upgraded from 4.7.6 to 4.7.11 to 5.0 then to 5.7. Then I decide to upgrade from 5.7 to 6.0.

When I did the update.php proper, I got a page saying register globals had to be disabled. So I searched this website and found out I had to add this to .htaccess: AddType x-mapp-php5 .ph.

I did this (I knew it worked cause I used it on a previous upgrade).

Then instead of clicking on the "continue button" I reloaded the page so the modified .htacesss file would be taken into account. Was that a mistake?

Then I got a lot of mysql warnings, and I thought perhaps reloading was the cause, so I ran update.php again (I've read you shouldn't do that but I didn't know what to do).

I got no error warnings, just the update button, so I cliked on it.

Then I got a lots of warnings. Here they are:

    * user warning: You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT vid FROM node n WHERE t.nid = n.nid)' at line 1 query: UPDATE term_node t SET vid = (SELECT vid FROM node n WHERE t.nid = n.nid) in /homepages/xxx/htdocs/modules/system/system.install on line 1154.
    * user warning: You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT n.uid FROM node n WHERE files.uid = n.nid)' at line 1 query: UPDATE files SET uid = (SELECT n.uid FROM node n WHERE files.uid = n.nid) in /homepages/xxx/htdocs/modules/system/system.install on line 1954.
    * user warning: You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT r.nid FROM node_revisions r WHERE upload.vid = r.vid)' a query: UPDATE upload SET nid = (SELECT r.nid FROM node_revisions r WHERE upload.vid = r.vid) in /homepages/xxx/htdocs/modules/system/system.install on line 1955.
    * user warning: Duplicate entry '384-0' for key 1 query: ALTER TABLE locales_target ADD PRIMARY KEY (language, lid, plural) in /homepages/xxx/htdocs/includes/database.mysql-common.inc on line 374.
    * user warning: Duplicate entry '67-0' for key 1 query: ALTER TABLE term_node ADD PRIMARY KEY (tid, vid) in /homepages/xxx/htdocs/includes/database.mysql-common.inc on line 374.
    * user warning: You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT lid FROM locales_target)' at line 1 query: DELETE FROM locales_source WHERE lid NOT IN (SELECT lid FROM locales_target) in /homepages/xxx/htdocs/modules/locale/locale.install on line 144.

At the bottom of the page, one failure:

The following queries were executed
....
Update #6003

    * Failed: DELETE FROM {locales_source} WHERE lid NOT IN (SELECT lid FROM {locales_target})

Our LUG website is hosted by 1&1.

I hope you can help me! I'm not familiar with MySQL apart from creating databases on the command line. I don't even know how to upgrade manually as Drupal recommends when there are errors. Yet, I can find out, I suppose.

Many thanks in advance :-)