The device was provisioned by using TP-Link Kasa iOS app. The firmware was not updated.
The app seems to work fine, but with kasa CLI the above error JSON_DECODE_FAIL_ERROR appears.

Tested for example with:

kasa --username username --password password --host 192.168.8.71 state

The --debug shows that handshaking is working (DEBUG:kasa.aestransport:Handshake with 192.168.8.71 complete), but further commands do not:

DEBUG:kasa.httpclient:Posting to http://192.168.8.71:80/app?token=93C323C226D276D274D25CD2D5D241D2
DEBUG:kasa.smartprotocol:192.168.8.71 multi-request-batch-1 << {'error_code': -1003}
DEBUG:kasa.smartprotocol:Unable to query the device: 192.168.8.71, not retrying: Error querying device: 192.168.8.71: JSON_DECODE_FAIL_ERROR(-1003) for method: multi-request-batch-1 (error_code=JSON_DECODE_FAIL_ERROR)