diff --git a/app/main.py b/app/main.py
index 8f26769..4c6a8ad 100644
--- a/app/main.py
+++ b/app/main.py
@@ -995,6 +995,18 @@ def set_member_matrix_mxid(email: str, mxid: str) -> None:
     conn.close()
 
 
+def _loomio_safe_username(raw) -> str | None:
+    """Sanitize a candidate into a Loomio-legal username.
+
+    Loomio accepts lowercase letters, numbers, underscores and INTERNAL hyphens
+    only, so a dot-containing candidate — an OAuth uid or an email local part
+    such as "joshua.z.tan" — must be normalized rather than passed through.
+    Passing it through is what made this fix fail with HTTP 422 on every sweep.
+    """
+    u = re.sub(r"[^a-z0-9_-]", "_", (raw or "").lower()).strip("-_")
+    return u if len(u) >= 2 else None
+
+
 async def loomio_fix_usernames(client: httpx.AsyncClient) -> dict:
     """Rename Loomio users with auto-generated handles to their Cloudron username.
 
@@ -1014,16 +1026,13 @@ async def loomio_fix_usernames(client: httpx.AsyncClient) -> dict:
         target = None
         for ident in u.get("identities") or []:
             if ident.get("identity_type") == "oauth" and ident.get("uid"):
-                uid = ident["uid"].lower()
-                # Loomio usernames allow lowercase letters, numbers, underscores and hyphens.
-                if uid.replace("_", "").replace("-", "").isalnum() and len(uid) >= 2:
-                    target = uid
-                    break
-                # uid unusable (dots etc) — use email local-part instead
-                target = (u.get("email") or "").split("@")[0].lower() or None
+                target = _loomio_safe_username(ident["uid"])
+                if not target:
+                    # uid unusable even after sanitizing — fall back to the email local part
+                    target = _loomio_safe_username((u.get("email") or "").split("@")[0])
                 break
-        if not target and u.get("email") and "@" in u["email"]:
-            target = u["email"].split("@")[0].lower()
+        if not target:
+            target = _loomio_safe_username((u.get("email") or "").split("@")[0])
         if not target or u.get("username") == target:
             skipped += 1
             continue
@@ -1034,8 +1043,9 @@ async def loomio_fix_usernames(client: httpx.AsyncClient) -> dict:
         if rr.status_code == 200:
             fixed += 1
         else:
-            logger.warning("Loomio username fix for %s failed: %s %s",
-                           u.get("email"), rr.status_code, rr.text[:120])
+            # log the Loomio user id, never the member's email
+            logger.warning("Loomio username fix for user %s failed: %s %s",
+                           u.get("id"), rr.status_code, rr.text[:120])
     if fixed:
         logger.info("Loomio usernames fixed: %s", fixed)
     return {"fixed": fixed, "skipped": skipped}