Hi.
You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') GROUP BY u.id' at line 4
The url look link this:
/category/author/ID-username?start=130
or
categroy/author/ID-username&format=feed&Itemid=467&type=rss&format=feed&Itemid=467&format=feed&Itemid=467?start=40

in url ó 
No matter if if turn on or off convert ó=o

Server version: 10.2.40-MariaDB 
Engine: InnoDB 

Error set on maximum show no errors php. 
Error logs show: Trying to get property 'alias' of non-object in *****/components/com_k2/router.php on line 292

Yes. 
screen all SEO setting. 
I don't have any other extension which could interfere. 
I noticed this thanks to Google Search Console.

I have created item in menu / username for clarity.

Ps. I have one suspect. But must verify.