MySQL issue

Hello,

With no changes on my part whatsoever, a few days ago, the following line (repeated multiple times) started to appear in the dashboard when I save updated menu:
Warning: strtolower() expects parameter 1 to be string, array given in /homepages/44/d186005129/htdocs/dmme/wp-includes/formatting.php on line 1870

A few days later, another message appeared – this time on the public-facing first page of my site, above the content (twice) and in the widget area (twice):
Warning: mysql_get_server_info(): No such file or directory in /homepages/44/d186005129/htdocs/dmme/wp-content/plugins/advanced-category-excluder/advanced-category-excluder.php on line 128

I had this plugin installed for me, as I’m not a coder; and I did try to deactivate and activate it. When it’s activated again, the message reads:
The plugin generated 426 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

Could you help me resolve this issue, please?