You could try a CVS build of dosbox, there you can set
usescancodes=true in dosbox.conf (might help).

Thanks for the advice. Any chance to get the CVS build anywhere but from http://cvscompile.aep-emu.de/dosbox.htm ? This page does not seem to work. But maybe, it's just some temporary problem - I will continue trying.

Try Ykhwong´s build. I think it's has been updated a few days ago.

CVS build with following in the dosbox.conf:

[sdl]
usescancodes=true

works perfectly. Thanks a lot.

Did you update SDL only, which broke the keyboard, or could it
be something else?

I was using dosbox from debian distribution. There was probably also some older libsdl version installed, but I'm not sure which one. Two days ago, I was compiling qemu from sources, because of some problems with debian version. For that, I needed development package of libsdl. Only one available was libsld1.2-dev, so I apt-got it. Apt-get removed some old sdl library (I don't remember which one) and installed whole bunch of libsdl1.2 packages. After that, Shift key in dosbox stopped working. I'm pretty sure I did not installed anything else.
Hope it helps. If you need some more info or help from my side, feel free to ask.

did your X upgrade as well ?

Definitely not. I'm using X.org, but it has been at least three weeks from last atp-get dist-upgrade (during which X were probably upgraded). I'm using dosbox almost each day now, so I discovered the problem immediately. I'm sure there was no other upgrade than sdl upgrade, between working-not working state.

Are you using a non-US keyboard layout in linux?
If so, maybe you could try to see what happens if
you switch it to the standard US one (should work
with usescancodes=false then as well).

I'm afraid this is not cause of my problem. I'm using three keyboard layouts in linux - US, Deutsch and Czech. The US layout is the default and I'm using it unless I'm creating some document.
However, to check it, I switched off the layouts in xorg.conf, so only US was available. However, the Shift key behavior is still the same, without "usescancodes=true".
I did some more testing and this is the description how exactly it works:

- without Shift pressed, all keys work as expected

- with Shift pressed, letter keys work (writing capitals). Also, keys / * - + on numerical keyboard do their job.

- no other "character" key works with Shift pressed (numbers on both normal and numerical keyboard and all special characters). nothing happens, when I press the keys.

- there is one strange exception, key ',', instead of producing '<' character produces '|' (pipe) character when Shift is pressed