I don't save index keywords in topics but a while back I took over a project where that was done. I found topic terms appeared in every index so I converted them.
If you decide to do that, go to Project Settings and make sure Add New Keywords is set to Index. Then open a topic and go to Topic Properties. The icon shows which keyword are in an Index and which are in the Topic. Click on one of the latter keywords and click Add. The same keyword gets added and then you delete the old one. Boring but quicker than other methods.
Sorry but I don't know why your index is failing. Maybe deleting the CPD at the same time as deleting the rhkeyword file is worth a try. Backup first though.
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Thanks Peter for your answer.
I already switched all index keywords in this project from topic to htt. I found an even quicker method to to this: select each keyword and press ALT+R (Replace command).
I also sent a bugbase to Adobe about the failing rhkkeyword.apj file and even got a reply asking for more details.
I tried to delete the CPD together with the fail file, but this did not return the CBT's on the index keywords. Actually, this empties the index.
About the choice wether to use topic based keywords or a HTT file: I used topic based, because if a topic is imported in another project, the index keywords are copied as well and the keywords can be edited in the html of the topic.