I extended the LV by 9G, which worked, unlike with -L +10G:

vega:$ sudo lvresize -L +9G vega/data
  Extending logical volume data to 89.00 GiB
  Logical volume data successfully resized

I then increased the size of the FS:

vega:$ sudo xfs_growfs /dev/mapper/vega-data
meta-data=/dev/mapper/vega-data  isize=256    agcount=6, agsize=3932160 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=20971520, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=7680, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 20971520 to 23330816

So far, so good. Now:

vega:$ sudo lvresize -L +1G vega/data
  Extending logical volume data to 90.00 GiB
  device-mapper: resume ioctl failed: Invalid argument
  Unable to resume vega-data (254:4)
  Problem reactivating data

+1G failed, so I then issued -L -1G:

vega:$ sudo lvresize -L -1G vega/data

lvresize froze at this command and went into uninterruptible sleep (status D). This is what happened before. Any further lvm commands such as lvdisplay, lvscan, vgdisplay also freeze in status D (verified with top). The only solution I can find is a system restart.