My problem was the same as yours.
My Nextcloud setup is:
NC - 15.0.5
Mysql -15.1 Distrib 10.1.38-MariaDB
PHP - 7.3
Apache2 - 2.4.38-2
I have fixed it by modifying the line listed below:
“sudo nano /etc/apache2/conf-available/http2.conf”
-H2StreamMaxMemSize 5120000000
+H2StreamMaxMemSize 128000
Save the file and restart apache2 service:
“sudo systemctl restart apache2”
I have followed the installation guide from link below, which is not available anymore.
https://www.c-rieger.de/nextcloud-installation-guide-apache2/
These links were helpfull for me:
https://datahunter.org/apache_http2
I hope it will help you.