Issue Summary

Calendar used to look up availability. When the ICS feed (in this case from Proton Calendar) contains a RECURRING event, availability on all days will be blocked by that recurring event.

It seems that DTSTART and DTEND are ignored.

Steps to Reproduce

Make sure the ICS-feed contains something like a 4 day event starting August 5 and recurring till August 9:

BEGIN:VEVENT
DTSTART;TZID=Europe/Amsterdam:20250805T080000
DTEND;TZID=Europe/Amsterdam:20250805T180000
RRULE:FREQ=DAILY;UNTIL=20250809T215959Z
SEQUENCE:1
ORGANIZER;CN=John Doe:mailto:john.doe@acme.eu
CREATED:20250520T194112Z
LAST-MODIFIED:20250520T194148Z
UID:097721F3-9E7A-43BB-9B7F-0563B0ECD92E@proton.me
DTSTAMP:20250725T074519Z
SUMMARY:SOMETHING TO DO ON HOLIDAYS
STATUS:CONFIRMED
ATTENDEE;CN=jane@acme.eu;RSVP=TRUE;X-PM-TOKEN=5a5d5b56ec47ea13
 c1c8b83590fcd1fec9c4f231;PARTSTAT=NEEDS-ACTION:mailto:jane@acme.eu
ATTENDEE;CN=jane2@acme.eu;RSVP=TRUE;X-PM-TOKEN=58fe53df38c8
 cc38e87f45c5dfb243db3c0bde0e;PARTSTAT=NEEDS-ACTION:mailto:jane2Acme.eu
END:VEVENT

ICS Source Calendar

Nothing on Sunday July 27.

All days are fully busy till August 28 (unclear why after that date, August 9 would have been more likely given the dependence on the recurring event)

Troubleshooter

Troubleshooter shows an every day event from 8 to 18, even on Sunday:

Please note that despite full details in the ICS feed, it only displays "Busy" which made it harder to analyze.

Removed recurring event

After changing the recuring event in Proton to 4 separate events, Cal.com again shows availability:

Workaround

When there is no availability, please check your ICS download for:

RRULE:FREQ=DAILY

and the same start and end times in DTSTART and DTEND. Replace those recurring calendar events by separate calendar events.