SignatureVerificationFailedException in WsFederationAuthenticationHandler with Azure

Question

I know very little about Azure and AD, so I need help. My users are logging on via the MS portal, the Azure control panel shows success against every log in users are being redirected to my auth sites 500 server error and this error is appearing in my logs.

System.IdentityModel.SignatureVerificationFailedException: ID4037: The key needed to verify the signature could not be resolved from the following security key identifier 'SecurityKeyIdentifier
(
IsReadOnly = False,
Count = 1,
Clause[0] = X509RawDataKeyIdentifierClause(RawData = MIIC4[...]3KtWG1BLD...'. 
Ensure that the SecurityTokenResolver is populated with the required key. 

at System.IdentityModel.EnvelopedSignatureReader.ResolveSigningCredentials()
   at System.IdentityModel.EnvelopedSignatureReader.OnEndOfRootElement()
   at System.IdentityModel.EnvelopedSignatureReader.Read()
   at System.Xml.XmlReader.ReadEndElement()
   at System.IdentityModel.Tokens.Saml2SecurityTokenHandler.ReadAssertion(XmlReader reader)
   at System.IdentityModel.Tokens.Saml2SecurityTokenHandler.ReadToken(XmlReader reader)
   at Microsoft.IdentityModel.Tokens.Saml2SecurityTokenHandler.ValidateToken(String securityToken, TokenValidationParameters validationParameters, SecurityToken& validatedToken)
   at Microsoft.IdentityModel.Extensions.SecurityTokenHandlerCollectionExtensions.ValidateToken(SecurityTokenHandlerCollection tokenHandlers, String securityToken, TokenValidationParameters validationParameters, SecurityToken& validatedToken)
   at Microsoft.Owin.Security.WsFederation.WsFederationAuthenticationHandler.<AuthenticateCoreAsync>d__1f.MoveNext()

Why has this stopped work??? I really don't know what else to offer as a clue, can anyone help me out?