This mostly sounds like a network issue (probably on myceliums's server side). This should be resolved within a day or two.
If you don't have time to (or don't want to) wait, you can always import your mnemonic seed into any BIP39 compatible wallet (e.g. electrum [1]).
The derivation path mycelium uses is:
m/44'/0'/X'/Y/Z, where:
X = Number of your account (i.e. for the 1st created 'account' inside mycelium: 0; for the 2nd: 1, ...)
Y = 0 for receiving addresses; 1 for change addresses
Z = index of address (0 for first addres, 1 for second addres, ...).
So in electrum you have to enter:
m/44'/0'/0'/ for your 'first account',
m/44'/0'/1'/ for your second account, ...
[1]
https://electrum.org/#home