I played again with Mitsumi USB FDD and I found the solution of the blue screen problem - to use DISKTSD.VXD 4.90.3000 and the old DISKTSD.VXD (4.10.2223) to rename to DISKTSD1.VXD. But not 100%.
Just replacing 4.10.2223 by 4.90.3000 does not work, the system won't boot and in the bootlog appears:
[00151BDA] Dynamic load device C:\WINDOWS\system\IOSUBSYS\disktsd.vxd
[00151BDA] Dynamic load failed C:\WINDOWS\system\IOSUBSYS\disktsd.vxd : [00151BDA] Not a valid device - possibly corrupt
After installing nusb, inserting the USB Floppy causes blues screen with error in DISKTSD.VXD.
After renaming old and copying new disktsd.vxd I was able to use the USB floppy with no problem.
BTW, in Windows Me everything works fine.
Then I tried to use 98SE2ME, at first the option installing just the USB drivers, second time option 2 - install everything.
In both cases plugging the USB floppy into the PC did not cause the blue screen, but in the device manager appeared yellow exclamation mark and the FDD did not work. The above described procedure with renaming and copying DISKTSD.VXD did not help.
I have shortly looked into the 4.90.3000 code and there are only very few differences in comparison to 4.10.2223 so it is possible that the 4.10.2223 version could be modified to work correctly - but I don't know how.
Has anybody successfully tested any USB Floppy with NUSB? Maybe the support should be completely removed and NUSB should not install its drivers for floppy USB, either in general, or at least for Mitsumi.
There is a driver supplied by Mitsumi, that could be added to NUSB. This driver contains (besides of INF files)
Maybe ot could work for other FDDs too?
I will try some additional testing tomorrow.