Random error is shown in Plesk interface:
'PLESK_ERROR: 'The license key is invalid. In order to use Plesk, please obtain and install a new valid license key. The license key is intended only for use with Plesk operating inside the following virtual environments: Virtuozzo containers, VMWare, Microsoft Hyper-V, Xen, KVM, Virtuozzo Server, LXC, Docker.
'PLESK_ERROR: 'Internal error: StatInfo->getProductVersion failed: sysinfo failed: Unable to execute the utility
/var/log/plesk/panel.log contains the following error:
ERR  /usr/local/psa/admin/bin/filwrpr ‘68’ failed with message: Unable to execute the utility. PHP Warning: proc_open(): fork failed - Cannot allocate memory; File: /usr/local/psa/admin/plib/common_func.php, Line: 227
Plesk considers failed execution of filwrpr as a hardware node instead of VPS. So when there is not enough memory on a server,
filwrpr would fail, causing license issue.
Add some memory on an instance by stopping the instance, changing it's type on Amazon management console, and starting an instance again. Make sure there is SWAP added too.