Bizarre happenings,
Normally it can happen if you are changing IP addresses, So if you are on your phone and switched from 3g to Wifi or from one wifi to another. etc
Do you tick the box Stay Logged in ?
For your desktop or Laptop , try clearing your browser cache including cookies. Then log in from scratch with the Stay logged in ticked, see how you get on
I will have a look at the software a bit later see if it's anything at this end as sunny suggested