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Sorry if this sounds stupid or a duplicate - I've tried to search for similar questions, but failed to troubleshoot my actual issue.

Thank you.

Here's what I see:
UPDATED
htop usage/analysis showed many hung application cron-launched processes. So, seems this is the cause of the memory usage increase. Looks like my next step is to find out why these processes are not being stopped.

SOLUTION
Finally, the issue was identified to be caused by a recently updated server library. The accepted answer is a good reminder, in the situation when you're stressed out by the memory usage, to trace back what had been changed in your system before the issue appeared.

Some tips I've been looking for and found to be very useful are described in How to find which processes are taking all the memory?

I'm listing below the commands that I used and may help in such situation.

ps auwx --sort rss - processes sorted by memory usage

ps -fu username - processes by a user

htop usage/analysis showed many hung application cron-launched processes in my case. I configured htop to output both PID and PPID, because I needed to correlate PPID to /var/log/cron logged processes.

Here's what I see: free -m

I don't see any memory high consuming processes using top/htop

Here's what I see: 
UPDATED
htop usage/analysis showed many hung application cron-launched processes. So, seems this is the cause of the memory usage increase. Looks like my next step is to find out why these processes are not being stopped.

free -m