Using b2 6.7.4 with PHP 7.X I received the following error:

Only variables should be passed by reference .../b2evolution/inc/links/model/_linkcache.class.php on line 290

This error appears consistently above the system status screen (above the b2evo menus) while in the back-office.

Once a blog is updated and the cache cleared, the error does not appear on the blog itself. However, after some times has elapsed, the same error as described above also appears on my blog. Suggesting to me is has something to do with the cache file.
I have system cache turned on, but no user cache installed.