When changing STP modes back to default it shows “Spanning tree enabled protocol ieee“.
The thing is that the ping between two hosts in different vlans with router on a stick configurations is not working anymore.

Off course, then I have checked the ping from the router to the hosts and they worked. I have turned to the first host, re-pinged the second host, and it worked.
The ping from second host to the first host was working since I have changed the spanning-tree to pvst.

SW3(config-if)#spa mode pvst
SW3(config)#do sh spa vl 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     001f.c958.1a80
             Cost        19
             Port        22 (FastEthernet0/22)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20)
             Address     0019.563a.9580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3            Desg LRN 19        128.3    P2p
Fa0/13           Desg LRN 19        128.13   P2p
Fa0/22           Root LRN 19        128.22   P2p

read here: https://learningnetwork.cisco.com/thread/35585