[cybmole]

here's my explanation -

I added some invalid extra css to my default conversion preferences - mismatched brackets.
as soon as I let that code lose on a conversion it corrupted settings elsewhere in calibre, I guess there's no validataion / protection surrounding extra css. So even after going to that default preferences screen and removing ALL extra CSS, the corruption was still present, and remained so after shut down+restart.

The cure of forcing every individual conversion screen back to default repaired the damage,even there was no VISIBLE damage on any other screen.
the reason that some conversions still worked was that those ones were re-using stored conversion preferences

i made two typo errors in extra css - like so
{mistake1}}
{mistake2]

somehow they corrupted a close by area like the search /replace values , in such a way that the corruption did not show up in the S&R window ???

someone who know how calibre is coded can maybe hypothesize what damage the mismatched brackets can do, without recreating a destructive test.

it was just con-incidence that the problem showed when I was fooling with lit conversions - I add "bright ideas" to my default extra CSS ( well not any more but I did! ) as I find them, without immediately testing. so I'd added a couple of things earlier in the day & introduced the 2 errors. but they lay dormant until I actually used my new defaults.