diff --git a/backend/webdav/webdav.go b/backend/webdav/webdav.go
index 1dd7b6060..ea25fa276 100644
--- a/backend/webdav/webdav.go
+++ b/backend/webdav/webdav.go
@@ -20,6 +20,7 @@ import (
 	"path"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/pkg/errors"
@@ -143,6 +144,7 @@ type Fs struct {
 	retryWithZeroDepth bool          // some vendors (sharepoint) won't list files when Depth is 1 (our default)
 	hasMD5             bool          // set if can use owncloud style checksums for MD5
 	hasSHA1            bool          // set if can use owncloud style checksums for SHA1
+	ntlmAuthMu         sync.Mutex    // mutex to serialize NTLM auth roundtrips
 }
 
 // Object describes a webdav object
@@ -206,6 +208,22 @@ func (f *Fs) shouldRetry(resp *http.Response, err error) (bool, error) {
 	return fserrors.ShouldRetry(err) || fserrors.ShouldRetryHTTP(resp, retryErrorCodes), err
 }
 
+// safeRoundTripper is a wrapper for http.RoundTripper that serializes
+// http roundtrips. NTLM authentication sequence can involve up to four
+// rounds of negotiations and might fail due to concurrency.
+// This wrapper allows to use ntlmssp.Negotiator safely with goroutines.
+type safeRoundTripper struct {
+	fs *Fs
+	rt http.RoundTripper
+}
+
+// RoundTrip guards wrapped RoundTripper by a mutex.
+func (srt *safeRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	srt.fs.ntlmAuthMu.Lock()
+	defer srt.fs.ntlmAuthMu.Unlock()
+	return srt.rt.RoundTrip(req)
+}
+
 // itemIsDir returns true if the item is a directory
 //
 // When a client sees a resourcetype it doesn't recognize it should
@@ -365,6 +383,16 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		return nil, err
 	}
 
+	f := &Fs{
+		name:        name,
+		root:        root,
+		opt:         *opt,
+		endpoint:    u,
+		endpointURL: u.String(),
+		pacer:       fs.NewPacer(ctx, pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
+		precision:   fs.ModTimeNotSupported,
+	}
+
 	client := fshttp.NewClient(ctx)
 	if opt.Vendor == "sharepoint-ntlm" {
 		// Disable transparent HTTP/2 support as per https://golang.org/pkg/net/http/ ,
@@ -374,19 +402,15 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		t := fshttp.NewTransportCustom(ctx, func(t *http.Transport) {
 			t.TLSNextProto = map[string]func(string, *tls.Conn) http.RoundTripper{}
 		})
+
 		// Add NTLM layer
-		client.Transport = ntlmssp.Negotiator{RoundTripper: t}
-	}
-	f := &Fs{
-		name:        name,
-		root:        root,
-		opt:         *opt,
-		endpoint:    u,
-		endpointURL: u.String(),
-		srv:         rest.NewClient(client).SetRoot(u.String()),
-		pacer:       fs.NewPacer(ctx, pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
-		precision:   fs.ModTimeNotSupported,
+		client.Transport = &safeRoundTripper{
+			fs: f,
+			rt: ntlmssp.Negotiator{RoundTripper: t},
+		}
 	}
+	f.srv = rest.NewClient(client).SetRoot(u.String())
+
 	f.features = (&fs.Features{
 		CanHaveEmptyDirectories: true,
 	}).Fill(ctx, f)