I finally gave up. I realized that maybe resize2fs would have done the trick after a test run with mke2fs -n showed me a much larger number of blocks that would be created if the partition was reformatted. resize2fs works only on unmounted partitions under kernels smaller than 2.6, and copying it to flash did not help as some other programs were probably needed.
I tried some other backup superblocks, but only those within the (wrong, smaller) block range were accepted.

So I had to do a backup as the drive is on a remote location and then reinstall the backup. Time-consuming, but in the end it worked out.

Merry Christmas,

F.