Since deadkeys are horribly annoying (German keyboard layout --> `, ^ and ´ are deadkeys) I've decided to use Microsoft Keyboard Layout Creator to create a custom keyboard layout to get rid of those deadkeys (as suggested in How to make ^ and ` non-dead-keys on Windows 7 with German keyboard layout).
This worked fine, however, as soon as I load a custom keyboard layout - even if it's completely unmodified (i.e. without the deadkey changes) - I cannot use CTRL+SHIFT+F (same for other keys, but F is the key where I had the problem first) anymore in the PuTTY terminal.
read shows me
^F when the original keyboard layout is enabled; when switching to a custom layout it does not receive anything when pressing CTRL+SHIFT+F - when just using CTRL+F it works fine though.