diff --git a/roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml b/roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml
index 1ae191e21..f00984a9d 100644
--- a/roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml
+++ b/roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml
@@ -4,9 +4,9 @@
 
 ---
 
-- name: Ensure Synapse user registered - {{ user.username | quote }}
-  ansible.builtin.command:
-    cmd: |-
+- name: Build Synapse user registration command - {{ user.username | quote }}
+  ansible.builtin.set_fact:
+    matrix_synapse_register_user_command: |-
       {{ devture_systemd_docker_base_host_command_docker }} exec matrix-synapse
       register_new_matrix_user
       -u  {{ user.username | quote }}
@@ -21,6 +21,31 @@
         {% endif %}
       {% endif %}
       http://localhost:{{ matrix_synapse_container_client_api_port }}
+
+- name: Ensure Synapse user registered - {{ user.username | quote }}
+  ansible.builtin.command:
+    cmd: "{{ matrix_synapse_register_user_command }}"
   register: matrix_synapse_register_user_result
   changed_when: matrix_synapse_register_user_result.rc == 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
-  failed_when: matrix_synapse_register_user_result.rc != 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
+  failed_when: >-
+    matrix_synapse_register_user_result.rc != 0
+    and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
+    and 'HMAC incorrect' not in matrix_synapse_register_user_result.stdout
+
+- when: "'HMAC incorrect' in matrix_synapse_register_user_result.stdout | default('')"
+  block:
+    - name: Restart Synapse due to HMAC failure (likely a registration_shared_secret change)
+      ansible.builtin.service:
+        name: "matrix-synapse.service"
+        state: restarted
+
+    - name: Wait for Synapse to start after restart
+      ansible.builtin.pause:
+        seconds: "{{ matrix_user_creator_homeserver_start_wait_time_seconds }}"
+
+    - name: Retry Synapse user registration - {{ user.username | quote }}
+      ansible.builtin.command:
+        cmd: "{{ matrix_synapse_register_user_command }}"
+      register: matrix_synapse_register_user_result
+      changed_when: matrix_synapse_register_user_result.rc == 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
+      failed_when: matrix_synapse_register_user_result.rc != 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout