After all the changes to Proofread Page Extension in 1.20wmf1 anon users can't change the quality of an page anymore. The main problem are the changes from proofreadPageUserName to mw.config.get( 'wgUserName' ) in proofreadpage.js. For anons the proofreadPageUserName was the IP (x.x.x.x)  but the wgUserName outputs null for IP users . This causes problems inside the proofreadpage_body.php because it blocks when the ip user performs his request. (Hopefully this is the only problem.)
Another problem is the position of the quality input boxes for anon users. See the proofreadpage.js  at the function pr_add_quality_buttons, specially at the lines 949-957 (with the extrem sibling action). Get the div with the class editCheckboxes and append the span child there is much easier and works for all usergroups (it is default set and for anon it is only empty).
Anon users have the right to change the quality of a page on de.ws per Bug 27516