I get this message when I try to upload a XLSX file. Used to work before in J4. Also still works when using cronjob. Just not when trying to upload manually.
{"error":true,"process":false,"url":"administrator\/index.php?option=com_csvi&view=templates","message":"Failed to start the session because headers have already been sent by xx/\/administrator\/components\/com_csvi\/assets\/vendor\/phpoffice\/phpspreadsheet\/src\/PhpSpreadsheet\/Style\/NumberFormat.php\" at line 476."}
Also the following message is in there many times:
Deprecated: Use of "self" in callables is deprecated in /xx/administrator/components/com_csvi/assets/vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Style/NumberFormat.php on line 502
I know it's not a real issue as it's just a deprecated warning, but it does make it harder to find the real errors. Perhaps easy to fix?
Cheers