CiviCRM exports displayed in browser – no download / header missing
If you have something like a file with a blank line at the top that might get output before the export data and cause the output to be interpreted as http content-type text instead of a download.
Check civicrm.settings.php or extensions or files in the CMS that might have changed.
You could also try running this from the root folder of your CMS:
find . -type f -exec awk 'NR==1&amp;&amp;/^$/{print FILENAME}' \{\} \;
or maybe this if the line isn't completely blank, like a couple tabs:
find . -type f -exec awk 'NR==1&amp;&amp;/^\s*$/{print FILENAME}' \{\} \;
https://askubuntu.com/a/1072314