diff --git a/main.go b/main.go
index 1050944..9269a5a 100644
--- a/main.go
+++ b/main.go
@@ -745,6 +745,7 @@ func (h *indexHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	sess, err := NewSession(w, r)
 	if err != nil {
+		log.Println("NewSession", err)
 		http.Error(w, err.Error(), 500)
 		return
 	}
@@ -805,6 +806,7 @@ func (h *indexHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		} else if format == "metakv" {
 			so, err := createStructuredFormat(mpPage)
 			if err != nil {
+				log.Println("createStructuredFormat", err)
 				http.Error(w, err.Error(), 500)
 				return
 			}
@@ -814,6 +816,7 @@ func (h *indexHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			enc.SetIndent("", "  ")
 			err = enc.Encode(so)
 			if err != nil {
+				log.Println("Encode", err)
 				http.Error(w, err.Error(), 500)
 			}
 			return
diff --git a/session.go b/session.go
index d11e8cb..9628ad3 100644
--- a/session.go
+++ b/session.go
@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"net/http"
 	"os"
+	"sync"
 	"time"
 )
 
@@ -40,12 +41,12 @@ type Session struct {
 func NewSession(w http.ResponseWriter, r *http.Request) (*Session, error) {
 	sessionID, err := getSessionCookie(w, r)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("getSessionCookie failed: %w", err)
 	}
 	session := &Session{ID: sessionID}
 	err = loadSession(session)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("loadSession failed: %w" , err)
 	}
 	return session, nil
 }
@@ -54,9 +55,13 @@ func (sess *Session) Flush() error {
 	return saveSession(sess)
 }
 
+var fileMutex sync.RWMutex
+
 func saveSession(sess *Session) error {
 	filename := generateFilename(sess.ID)
 	err := os.Mkdir("session", 0755)
+	fileMutex.Lock()
+	defer fileMutex.Unlock()
 	f, err := os.Create(filename)
 	if err != nil {
 		return err
@@ -69,17 +74,22 @@ func saveSession(sess *Session) error {
 func loadSession(sess *Session) error {
 	filename := generateFilename(sess.ID)
 	err := os.Mkdir("session", 0755)
+	fileMutex.RLock()
+	defer fileMutex.RUnlock()
 	f, err := os.Open(filename)
 	if err != nil {
 		if os.IsNotExist(err) {
 			// add defaults to session?
 			return nil
 		}
-		return err
+		return fmt.Errorf("while opening file %s: %w", filename, err)
 	}
 	defer f.Close()
 	err = json.NewDecoder(f).Decode(sess)
-	return err
+	if err != nil {
+		return fmt.Errorf("while decoding json from file %s: %w", filename, err)
+	}
+	return nil
 }
 
 func generateFilename(id string) string {