An error has occurred: 0 [] operator not supported for strings

Sometime between Aug 23. and Sept 11. the following issue started: Selecting any menu item to J!mailAlerts (JmailAlerts, Frequencies, Sync,...) leads to a

An error has occurred.

0 [] operator not supported for strings

page, screenshot here:
https://screenshots.firefox.com/rbunBLX2IKnSNkt9/svjsvetova523-1.cz

JMA has been installed new (over the existing installation), but that did not help. Any thoughts, please, what should I check?

Joomla 3.8.12, JMailAlerts 2.5.4