Sorry for any confusion there Maemish...
I know some folks may throw around the usage of force install casually...
But if something doesn't install right in the first place the proper way ..
Taking a wrong puzzle piece and a mallet and beating a piece into place that never was meant to fit that way...
Is perhaps a better imagery to use.

I don't force install anything ....personally.
nor do I leave devel or development enabled while issuing terminal commands to update fix missing ...

I can't be 100% sure...
Since you haven't given a blow by blow account of everything exactly done...from the moment of reflash.
(Personally I video and say what I am doing next ...then pause it until the next step of whatever I am doing next...it is easier than writing it down...and better than audio...and I can flip through it to see what I did where and narrow down cause and effect...and be certain what the cause and effect are ...)

And I don't gamble unless the odds are definitely in my favour..
and Maemish man ...leaving devel /development enabled and doing what you have done with "fix missing" ...and goodness knows what else you have done before that.... not documented ...
you said it yourself ...
"I don't know what was broken now or how to either fix it or prevent it from happening again"
I can point to devel and development still enabled while playing with term as a certainty of the reason.
Definitely I could point to "cause and effect" here.

If you also do not know exactly what caused this ..it also means a documentation issue ...
if you go off site and do stuff that no one else does or isn't normally prescribed methods or is experimental and there isn't much info on cause and effect except warnings....
You need to be thorough in documentation .
I know you know how to reflash already.
That isn't the point.

I suggest you reflash and start over with exacting documentation ..
It is easier to hit the record button every time you go to do something... than writing it down ....or not recording it at all...
and test the n900 in between ....every experimental thing you are doing...
to make sure you nip issues where the start.

and it isn't exactly a desirable thing to do...reflash...reflash and reflash again..
But when experimenting ...one gets used to doing it over and over ...while working on a puzzle...so you can replicate the failure using a clean and documented device...as much as replicate the success using a clean and documented device.