Hello marcinbb,
Please find my answers below -
1. I checked and found the defect is fixed in PowerCLI 5.0 build 435427. Internal ref. 717374.
1.1. I would recommend that you deploy the latest product version. The application is backwards compatible: PowerCLI v5.1 R2 has been cleared for release in an ESXi 5.0.x environment.
2. The Global Support Services team is best positioned to examine your environment, analyse the issue, and propose a workaround, if one is available. We work closely together on support requests, providing engineering support when necessary. (I am from the PowerCLI Engineering group).
I hope this helps,
Hristo