For security reasons I don't install bitcoin-qt in standard directoy and use a none standard datadir.
My bitcoin-qt is installed as c:\abc\bitcoin\bitcoin-qt.exe . My datadir is f:\def\btc so the .conf file is f:\def\btc\bitcoinqt.conf
I define
datadir=f:\def\btc
c:\abc\bitcoin\bitcoin-qt -conf=f:\def\btc\bitcoinqt.conf
After upgrading from v0.8.6 to v0.9.1 bitcoin-qt it either doesn't start (hangs there forever)
If I start v0.9.1 bitcoin-qt without -conf= it starts fine. It shows that at least the installation is correct.
This seems to be a bug. It breaks drop-in substitution with the v0.9.1 bitcoin-qt. I posted in the tech support forum but didn't get a meaningful answer.