How does the error manifest?

Every time the cronjob with cliupdate.php is running and new entries in the rss feed I mentioned are fetched.

Is it surronded by other log statements or just interspersed.

No. I only get an E-Mail from the cronjob where the error is included

Does it crash the PHP process?

I don't know.
I am running selfoss on a shared host and I can not see what happens with the PHP-process.

What PHP, libpng and distribution versions do you use?

PHP 7.2.13
libpng 1.6.28
Debian with Kernel 4.9.xxx (shared host, no own server)