I've turned on the debugger and I think I found the problem. The query when filtering on just an event, without selecting a category is:

SELECT det.evdet_id as detailid, rpt.*, ev.*, rr.*, det.* ,  ev.state as published, ev.created as created , loc.loc_id,loc.title as loc_title, loc.title as location, loc.street as loc_street, loc.description as loc_desc, loc.postcode as loc_postcode, loc.city as loc_city, loc.country as loc_country, loc.state as loc_state, loc.phone as loc_phone	, loc.image as loc_image	, loc.url as loc_url	, loc.geolon as loc_lon	, loc.geolat as loc_lat	, loc.geozoom as loc_zoom	, loc.mapicon as loc_mapicon	, GROUP_CONCAT(DISTINCT loccat.title SEPARATOR ',') AS location_category_list,GROUP_CONCAT(loccatmap.catid SEPARATOR ',') AS location_catid_list                , jfc.imagename1 AS imageurl1, jfc.imagetitle1 AS imagetitle1, jfc.imagecomment1 AS imagecomment1                , CASE WHEN (jfc.filename1='') THEN '' ELSE CONCAT('/images/jevents/',jfc.filename1) END as filehref1

                , jfc.filetitle1 as filetitle1

                , CASE WHEN (jfc.filename1='') THEN '' ELSE CONCAT('<a href="/images/jevents/',jfc.filename1,'" />',jfc.filetitle1,'</a>') END as filelink1, GROUP_CONCAT(DISTINCT catmapcat.id ORDER BY catmapcat.lft ASC SEPARATOR ',' ) as catids

 , YEAR(rpt.startrepeat) as yup, MONTH(rpt.startrepeat ) as mup, DAYOFMONTH(rpt.startrepeat ) as dup

 , YEAR(rpt.endrepeat  ) as ydn, MONTH(rpt.endrepeat   ) as mdn, DAYOFMONTH(rpt.endrepeat   ) as ddn

 , HOUR(rpt.startrepeat) as hup, MINUTE(rpt.startrepeat ) as minup, SECOND(rpt.startrepeat ) as sup

 , HOUR(rpt.endrepeat  ) as hdn, MINUTE(rpt.endrepeat   ) as mindn, SECOND(rpt.endrepeat   ) as sdn

 FROM si3ow_jevents_repetition as rpt

 INNER JOIN si3ow_jevents_vevent as ev ON rpt.eventid = ev.ev_id

 INNER JOIN si3ow_jevents_icsfile as icsf ON icsf.ics_id=ev.icsid 

 INNER JOIN si3ow_jevents_vevdetail as det ON det.evdet_id = rpt.eventdetail_id

 LEFT JOIN si3ow_jevents_rrule as rr ON rr.eventid = rpt.eventid 

 LEFT JOIN  si3ow_jev_tageventsmap as tagmap ON tagmap.ev_id = ev.ev_id  

 LEFT JOIN  si3ow_jev_locations as loc ON loc.loc_id=det.loc_id 

 LEFT JOIN  si3ow_jevlocations_catmap AS loccatmap ON loccatmap.locid = loc.loc_id 

 LEFT JOIN  si3ow_categories AS loccat ON loccat.id = loccatmap.catid 

 LEFT JOIN  si3ow_jev_files_combined as jfc ON jfc.evdet_id = det.evdet_id 

 LEFT JOIN  si3ow_jev_peopleeventsmap as persmap ON det.evdet_id=persmap.evdet_id LEFT JOIN si3ow_jev_people as pers ON pers.pers_id=persmap.pers_id  

 LEFT JOIN 

 si3ow_jevents_catmap as catmap ON catmap.evid = rpt.eventid 

 LEFT JOIN 

 si3ow_categories AS catmapcat ON catmap.catid = catmapcat.id

 WHERE 1 

 AND rpt.rp_id in (SELECT rptx.rp_id FROM si3ow_jevents_repetition as rptx 

 INNER JOIN si3ow_jevents_vevdetail as det2 ON det2.evdet_id = rptx.eventdetail_id

 WHERE  

 rptx.endrepeat >= '2022-07-05 00:00:00' AND rptx.startrepeat <= '2022-07-05 23:59:59' 

 AND NOT (rptx.startrepeat < '2022-07-05 00:00:00' AND det2.multiday=0) ) AND ev.state=1 AND (det.summary LIKE '%Marijn%' OR det.description LIKE '%Marijn%' OR det.extra_info LIKE '%Marijn%') AND (catmap.catid NOT IN (383,540,544,543,537,387,388,389,390,391) OR (catmap.catid IN (383,540,544,543,537,387,388,389,390,391) )) AND  catmap.catid IN(-1)

 AND ev.access IN (1,10)  AND icsf.state=1 AND icsf.access IN (1,10)

 GROUP BY rpt.rp_id

I see two things here:
First, the reason it's not finding anything, is the condition catmap.catid IN(-1) It should be left out, if there's no category selected. (If there is a category selected, the where condition is catmap.catid IN(-1,391). Still I don't see a reason why there's a search for -1)
Second, the condition (catmap.catid NOT IN (383,540,544,543,537,387,388,389,390,391) OR (catmap.catid IN (383,540,544,543,537,387,388,389,390,391) )) is quite strange. On the one hand it saying not to search for all categories and then it is saying to do search in all categories.

I'll check this on the dev site. I half recall resolving an issue like this recently - I'll double check that everything is up to date.

I found the cause of the problem and have fixed it on your sandbox site with this build of JEvents.

We had hardened the category filter search to trap for potential security issues which converted a blank category into a zero which in turn lead to this issue . the new code traps blank filter values specifically which avoids this issue

Thanks for highlighting the problem