Sounds like a good idea. In the future, I will configure them in fixed

mode, it won't hurt.
Even though it may have something to do with the autoselect mode,

the whole story still has a bad smell =) I mean, the problem persisted

even though I several times disconnected the cable, changed the cable,

hooked the Soekris to a Dell 5324 Switch (to a port in autoselect mode)

and nevertheless the interface did not get out of the stalled state

until I did the ifconfig down/up sequence.