I encounter the same issue.
Using the long-term database to query 30 days results in "An unknown error occurred with loading the data" and
(mod_fastcgi.c.421) FastCGI-stderr: PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 16777224 bytes) in /var/www/html/admin/api_db.php on line 148
in /var/log/lightttp/error.log
I'm running vanilla armbian on NanoPi Neo Plus 2 (1GB RAM, 128M were allowed for php) with only Pihole installed and running. I have a small setup with max 15 clients and <15k queries a day.
Changing memory_limit in /etc/php/7.3/cgi/php.ini to 512M and
sudo service lighttpd force-reload
solved the issue partially.
Querying 30 days gives now 442.000 queries, this year and all time failed despite of 512M ram.
I also saw that only 1 CPU is used during processing.
I'm aware this issue is due to memory constrains but is it possible to change the warning in the WEB GUI to something more meaningful ("Error: Not enough RAM allocated to PHP")? - the issue likely will pop up more often when v5.0 is released.