It appears that the sudden delays and timeouts occur sometimes due to a glitch in mgmt-vmware service.
I had a backup problem (couldnt create snapshot) - the message was "another task is already in progress" but NO task was really in progress.
I tried to even shut down the virtual machine and nothing worked.
I noticed that's not the only machine with this kind of problems, and all machines with problems had in common that they run on a particular esx host.
The solution (found all around the internet through alot of blogs) is:
a) connect to esx host (ssh / putty)
b) stop mgmt-vmware service -> service mgmt-vmware stop (your esx host will look "disconected" from virtual center... dont panic)
c) wait at least 15 seconds
d) start mgmt-vmware service -> service mgmt-vmware start
d) recconect the esx host in vcenter
This worked for me.
Hope it will work for others too.
PS: Dont panic, the machines that run on that esx host will not be affected.