Can't login after changing the base currency, Store standard currency is EUR.

I have registered a customer with base currency EUR. And after I change the base currency to GBP i cant login with that customer account, its keeps loading but nothing comes. After a while it throws an error Service Temporarily Unavailable. But the same user can login when the base currency is EUR . Problem is with switching currency.

Same thing hapens when login with base currency EUR en when logged in switch to GBP or other currency we get the same error.

After that have to delete cache and cookies to get back to site.

Basically we are trying to implement Currency switcher, it works well for guest users. But doesn't work when the user is trying to login and the current currency does not match to the currency at the time of user registration.

magento version is 1.9.3.4 CE Theme Ultimo