diff --git a/clientapi/auth/sso/oauth2.go b/clientapi/auth/sso/oauth2.go
index 4d62ce3a6..cd6186938 100644
--- a/clientapi/auth/sso/oauth2.go
+++ b/clientapi/auth/sso/oauth2.go
@@ -26,6 +26,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/setup/config"
 	uapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/util"
 	"github.com/tidwall/gjson"
 )
 
@@ -130,29 +131,17 @@ func (p *oauth2IdentityProvider) getAccessToken(ctx context.Context, callbackURL
 	hreq.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	hreq.Header.Set("Accept", p.responseMimeType)
 
-	hresp, err := p.hc.Do(hreq)
+	hresp, err := httpDo(ctx, p.hc, hreq)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("access token: %w", err)
 	}
 	defer hresp.Body.Close() // nolint:errcheck
 
-	if hresp.StatusCode/100 != 2 {
-		return "", fmt.Errorf("OAuth2 access token request %q failed: %d %s", p.accessTokenURL, hresp.StatusCode, hresp.Status)
-	}
-
 	var resp oauth2TokenResponse
 	if err := json.NewDecoder(hresp.Body).Decode(&resp); err != nil {
 		return "", err
 	}
 
-	if resp.Error != "" {
-		desc := resp.ErrorDescription
-		if desc == "" {
-			desc = resp.Error
-		}
-		return "", fmt.Errorf("failed to retrieve OIDC access token: %s", desc)
-	}
-
 	if strings.ToLower(resp.TokenType) != "bearer" {
 		return "", fmt.Errorf("expected bearer token, got type %q", resp.TokenType)
 	}
@@ -163,10 +152,6 @@ func (p *oauth2IdentityProvider) getAccessToken(ctx context.Context, callbackURL
 type oauth2TokenResponse struct {
 	TokenType   string `json:"token_type"`
 	AccessToken string `json:"access_token"`
-
-	Error            string `json:"error"`
-	ErrorDescription string `json:"error_description"`
-	ErrorURI         string `json:"error_uri"`
 }
 
 func (p *oauth2IdentityProvider) getUserInfo(ctx context.Context, accessToken string) (subject, displayName, suggestedLocalpart string, _ error) {
@@ -177,16 +162,12 @@ func (p *oauth2IdentityProvider) getUserInfo(ctx context.Context, accessToken st
 	hreq.Header.Set("Authorization", "Bearer "+accessToken)
 	hreq.Header.Set("Accept", p.responseMimeType)
 
-	hresp, err := p.hc.Do(hreq)
+	hresp, err := httpDo(ctx, p.hc, hreq)
 	if err != nil {
-		return "", "", "", err
+		return "", "", "", fmt.Errorf("user info: %w", err)
 	}
 	defer hresp.Body.Close() // nolint:errcheck
 
-	if hresp.StatusCode/100 != 2 {
-		return "", "", "", fmt.Errorf("OAuth2 user info request %q failed: %d %s", p.userInfoURL, hresp.StatusCode, hresp.Status)
-	}
-
 	body, err := ioutil.ReadAll(hresp.Body)
 	if err != nil {
 		return "", "", "", err
@@ -212,6 +193,56 @@ func (p *oauth2IdentityProvider) getUserInfo(ctx context.Context, accessToken st
 	return
 }
 
+func httpDo(ctx context.Context, hc *http.Client, req *http.Request) (*http.Response, error) {
+	resp, err := hc.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode/100 != 2 {
+		defer resp.Body.Close()
+
+		contentType := resp.Header.Get("Content-Type")
+		switch {
+		case strings.HasPrefix(contentType, "text/plain"):
+			bs, err := ioutil.ReadAll(resp.Body)
+			if err == nil {
+				if len(bs) > 80 {
+					bs = bs[:80]
+				}
+				util.GetLogger(ctx).WithField("url", req.URL.String()).WithField("status", resp.StatusCode).Warnf("OAuth2 HTTP request failed: %s", string(bs))
+			}
+		case strings.HasPrefix(contentType, "application/json"):
+			// https://openid.net/specs/openid-connect-core-1_0.html#TokenErrorResponse
+			var body oauth2Error
+			if err := json.NewDecoder(resp.Body).Decode(&body); err == nil {
+				util.GetLogger(ctx).WithField("url", req.URL.String()).WithField("status", resp.StatusCode).Warnf("OAuth2 HTTP request failed: %+v", &body)
+			}
+			if body.Error != "" {
+				return nil, fmt.Errorf("OAuth2 request %q failed: %s (%s)", req.URL.String(), resp.Status, body.Error)
+			}
+		}
+
+		if hdr := resp.Header.Get("WWW-Authenticate"); hdr != "" {
+			// https://openid.net/specs/openid-connect-core-1_0.html#UserInfoError
+			if len(hdr) > 80 {
+				hdr = hdr[:80]
+			}
+			return nil, fmt.Errorf("OAuth2 request %q failed: %s (%s)", req.URL.String(), resp.Status, hdr)
+		}
+
+		return nil, fmt.Errorf("OAuth2 HTTP request %q failed: %s", req.URL.String(), resp.Status)
+	}
+
+	return resp, nil
+}
+
+type oauth2Error struct {
+	Error            string `json:"error"`
+	ErrorDescription string `json:"error_description"`
+	ErrorURI         string `json:"error_uri"`
+}
+
 func resolveURL(urlString string, defaultQuery url.Values) (*url.URL, error) {
 	u, err := url.Parse(urlString)
 	if err != nil {