It would be worth getting a diagnostic done too to see what triggered that warning in iDrive. It would seem to be too much of a coincidence.
The recommended interval for changing brake fluid is 2 years. Over time it can absorb moisture and this can ruin seals within the system as well as lead to deteriorated braking performance. Moisture can get in via the flexible brake hoses or reservoir cap. Rubber is porous, even when new.
Your issue seems to have come on suddenly so it may not necessarily be related to the lack of fluid changes. Was it all four wheels that locked up? Or could it have been just one wheel? If all four then as others have said it could be a failing master cylinder, if it was only one wheel it could be a sticking calliper.