About two weeks in advance of the merger of Starwood and Marriott’s schemes, Amex in the UK has suspended transfers to the Starwood scheme, SPG. The normal transfer rate in the UK was 2 Membership Rewards points to 1 Starpoint. Whilst the option remains listed there is no ‘Transfer my points’ button to undertake the exchange.
Amex indicate that transfers will resume on 21 August 2018.
I can also confirm that transfers are also suspended for us AMEX MR users here in France :
https://catalogue.membershiprewards.fr/viewpartneroverview.mtw?currenttravelproducttype=Hotel&categoryname=fr_2_c_lodging&linknav=fr-mr-vrp-shoppinglanding-travelmenu-fr2clodging