Server with mariadb sometimes dies under high load
I assume you hit the open_files_limit. This should be indicated somewhere further up of your warning about the instances in your MariaDB Error Log.
The problem here is, that MariaDB behaves differently than MySQL:
In MySQL the number of FD (open_files_limit) used is: table_open_cache x 2 + ...
In MariaDB the number of DD used is table_open_cache x table_open_cache_instances x 2 + ...
https://mariadb.com/kb/en/server-system-variables/#table_open_cache_instances
Why they made it different I do not know.
So in your case you have:
table_open_cache = 207000 x table_open_cache_instances = 8
= requiring 1.6M TOC entries but you only have defined 207k as open_files_limit = 207000
So either lower the table_open_cache by at least 8 times or increase the open_files_limit by at least 8 times.
3 additional thoughts:

Do you really need 200k or 1.6M FD??? That happens only very rarely when you have multi-tenant set-ups or 100ks of tables.

See also here: https://fromdual.com/mysql-shared-hosting-configuration

Files are configured today in SystemD: https://fromdual.com/node/1270#systemd

The effective number of FD (open_files_limit) you need is calculated in:



MariaDB: table_open_cache x table_open_cache_instances x 2 + max_connections + 40
MySQL: table_open_cache x 2 + max_connections + 10

Regards,
Oli