diff --git a/backend/caldav_client.py b/backend/caldav_client.py
index 1268413..69ac449 100644
--- a/backend/caldav_client.py
+++ b/backend/caldav_client.py
@@ -4,7 +4,11 @@ from datetime import date, datetime, timedelta, timezone
 from typing import Dict, List, Optional
 
 import caldav
-from icalendar import Calendar, Event, vDatetime, vRecur
+from icalendar import Calendar, Event, vDatetime
+
+# One parser for both directions: this file used to carry its own copy with
+# the same UNTIL defect.
+from ical_io import _rrule_str_to_vrecur as _parse_rrule_str
 
 logger = logging.getLogger(__name__)
 
@@ -306,20 +310,6 @@ def delete_event(url: str, username: str, password: str, event_url: str,
     resource.delete()
 
 
-def _parse_rrule_str(rrule_str: str) -> vRecur:
-    """Parse an RRULE string like 'FREQ=WEEKLY;BYDAY=MO,WE' into a vRecur."""
-    params = {}
-    for part in rrule_str.split(";"):
-        if "=" not in part:
-            continue
-        key, val = part.split("=", 1)
-        if "," in val:
-            params[key] = val.split(",")
-        else:
-            params[key] = val
-    return vRecur(params)
-
-
 def _parse_dt(s: str) -> datetime:
     s = s.replace("Z", "+00:00")
     dt = datetime.fromisoformat(s)
diff --git a/backend/ical_io.py b/backend/ical_io.py
index 6574fb6..694a530 100644
--- a/backend/ical_io.py
+++ b/backend/ical_io.py
@@ -135,13 +135,17 @@ def parse_ics(raw: bytes) -> dict:
 
 
 def _rrule_str_to_vrecur(rrule_str: str) -> vRecur:
-    params = {}
-    for part in rrule_str.split(";"):
-        if "=" not in part:
-            continue
-        key, val = part.split("=", 1)
-        params[key] = val.split(",") if "," in val else val
-    return vRecur(params)
+    """Parse an RRULE string into a vRecur with correctly typed values.
+
+    Hand-splitting on ";" and "=" leaves every value a str, which icalendar
+    accepts silently here and then rejects inside ``to_ical()``: vRecur.types
+    maps UNTIL to vDDDTypes, and that demands a date. A single "repeats until
+    <date>" event therefore made the whole calendar fail to serialise — a 500
+    on REPORT, on GET and on export, while PROPFIND kept working because it
+    never builds ICS. Letting icalendar parse its own format gives UNTIL as a
+    datetime and COUNT/INTERVAL as ints.
+    """
+    return vRecur(vRecur.from_ical(rrule_str))
 
 
 def _parse_iso(s: str) -> datetime:
@@ -198,7 +202,10 @@ def build_ics(calendar, events, *, name_cache: dict | None = None,
         if ev.color:
             item.add("x-calendarr-color", ev.color)
         if ev.rrule:
-            item.add("rrule", _rrule_str_to_vrecur(ev.rrule))
+            try:
+                item.add("rrule", _rrule_str_to_vrecur(ev.rrule))
+            except (ValueError, TypeError):
+                logger.warning("Skipping unparsable RRULE %r on event %s", ev.rrule, ev.id)
 
         # ORGANIZER from the creator (local user or imported name).
         organizer_name = None
diff --git a/backend/tests/test_ical_rrule.py b/backend/tests/test_ical_rrule.py
new file mode 100644
index 0000000..0186d09
--- /dev/null
+++ b/backend/tests/test_ical_rrule.py
@@ -0,0 +1,52 @@
+"""RRULE serialisation.
+
+An RRULE whose values stayed strings serialised fine until ``to_ical()``, where
+icalendar rejects a str for UNTIL. One "repeats until <date>" event therefore
+returned 500 for every REPORT, GET and export of its calendar while PROPFIND
+kept succeeding — which is how a Home Assistant calendar ends up discovered
+under the right name and permanently unavailable.
+"""
+
+import pytest
+
+from ical_io import _rrule_str_to_vrecur
+
+
+@pytest.mark.parametrize("rule", [
+    "FREQ=WEEKLY;UNTIL=20260501T235959Z",
+    "FREQ=DAILY;UNTIL=20260501T235959Z;INTERVAL=2",
+    "FREQ=WEEKLY;BYDAY=MO,WE;UNTIL=20260501T235959Z",
+])
+def test_until_survives_serialisation(rule):
+    """The regression: these used to raise inside to_ical()."""
+    out = _rrule_str_to_vrecur(rule).to_ical().decode()
+    assert "UNTIL=20260501T235959Z" in out
+
+
+@pytest.mark.parametrize("rule", [
+    "FREQ=WEEKLY;BYDAY=MO",
+    "FREQ=DAILY;COUNT=5",
+    "FREQ=WEEKLY;BYDAY=MO,WE,FR;INTERVAL=2",
+    "FREQ=MONTHLY;BYMONTHDAY=15",
+])
+def test_rules_without_until_still_round_trip(rule):
+    """These always worked; pin them so the fix did not narrow what is accepted."""
+    out = _rrule_str_to_vrecur(rule).to_ical().decode()
+    for part in rule.split(";"):
+        key, val = part.split("=", 1)
+        assert key in out
+        for v in val.split(","):
+            assert v in out
+
+
+def test_count_and_interval_become_integers():
+    """icalendar types these; the hand-rolled parser left them as strings."""
+    rec = _rrule_str_to_vrecur("FREQ=DAILY;COUNT=5;INTERVAL=2")
+    assert rec["COUNT"] == [5]
+    assert rec["INTERVAL"] == [2]
+
+
+def test_malformed_rule_raises_rather_than_corrupting():
+    """build_ics catches this and drops the recurrence; it must not pass silently."""
+    with pytest.raises((ValueError, TypeError)):
+        _rrule_str_to_vrecur("FREQ=WEEKLY;UNTIL=not-a-date").to_ical()