Need HELP

I have 4 Domains at the same Server.

fahrzeugbeklebung.center
fraesbuchstaben.center
lichtwerbung.center

and they are showing only this:
Warning: Cannot modify header information – headers already sent by (output started at /kunden/107405_60311/rp-hosting/10002/20002/wp/wordpress/wp-content/plugins/ewww-image-optimizer/common.php:391) in /kunden/107405_60311/rp-hosting/10002/20002/wp/wordpress/wp-content/mu-plugins/domain_mapping.php on line 708

at the page logo-werbung.info (mainpage) is nearly everything working. but if i try to open the 3 href Pic and the href Fahrzeugbeschriftung and Schaufenster und Fassaden on the page http://www.logo-werbung.info/startseite/beschriftung/ i get back this:
http://www.logo-werbung.info/wp-signup.php?new=logo-werbung.info

maybe enybody can help me to fix the problems?!

THX
Simon

https://wordpress.org/plugins/wordpress-mu-domain-mapping/