So, I just talked to support again. Despite support having told me multiple times previously that I'm either on a 150GB home capped or 150GB business capped account, and despite me having used mroe than 100GB of data outside my free time last month, support this time told me that I'm still on the 100GB Business Standard Capped account. I also got told that the old accounts still exists, they just don't advertise them anymore.
Support now says they moved me over to the 150GB Business Hybrid Capped account. With this move, daily usage emails should start working for me again, they say. Let's hope that this saga is finally over now.
What I'm guessing happened is that I automatically got moved over to the Crystal Web Xtreme capped product, since that's what my usage emails tended to indicate, and also what support (mostly) told me I was on. When I contacted billing, who then contacted management, I guess they realised the mistake and moved me back to the 100GB Business Standard Capped package that I was previously on - and then the next daily usage email I got was broken again.