After I've completed the main-questline for Dawnguard, I had Serana 'wait here' at a purchased house. But than after doing some exploring and other things in less than an hour, at one point I go to Fort Dawnguard and find her there, so I try to speak with her but none of the dialogue options that use to come up..show up anymore, such as having her follow you, part ways, etc.. and now she just repeats "Yes?" and several other phrases.

Any fixes to this?

Load a save before you asked her to wait? And then do not ask her to wait.

I have had that happen.... And now she is working again.

I had a save from just after the end of the MQ, before we left Volkihar island that I loaded up, and Seana was not glitched. I found that she stayed fine after that. Weirdly, I was able to have a second follower.

Serana stayed glitch free and the follow and dismiss commands are still functioning ok.

I should mention that when she was 'broken', I had sent her to be cured immediately after the MQ. When I went back and reloaded the old save, I left her as a vamp. As a lark, after Aela, Serana and I did a few weeks of questing, I had the cure dialog, and off she went. When next we met, it broke the ability to have two followers.... But Serana herself still responds to the wait and follow commands properly.

My point in all this is that if you find a save where she works, start over from there, and you may be lucky and she won't break again... And I am not sure if curing her does not contribute to her issues. Experiment....