It looks like the 3.0 (rev 150) was broken for summaries - a last minute typo and I did not spot the mistake in the output. I have uploaded a rev 151 which fixes that. Thanks for pointing that out before too many people got the bad version.
If rev 151 is not working for you, then please either PM me a link to your Dropbox library (do not post it here for everyone to see) or raise an issue on the
calibre2opds Issue register so we can investigate further/
Note that the value in the GUI does not affect the length of the summary in the Book Details pane. It only affects the length of the summary displayed when getting lists of books. A value in the range 100-250 for the maximum summary length normally makes sense on those pages.