Virtuemart SEO and Joomla Cache

[Juraj]

Hi,
when i use this settings:

Joomla configuration
Search Engine Friendly URLs: Yes
Use Apache mod_rewrite: Yes
Add suffix to URLs: No
Cache: Yes
Cache Time: 15 Minutes
Cache Handler: File

VirtueMart configuration
SEO Disabled: No

on

Joomla: 1.5.26
VirtueMart: 2.0.6
Server OS: Ubuntu 12.04
PHP: 5.3.10
MySQL: 5.5.24

VM doesn't work and display this error:

Catchable fatal error: Object of class stdClass could not be converted to string in /XXX/libraries/joomla/cache/storage/file.php on line 88

When i disable cache in Joomla, or SEO in VM everything is OK - but this is not a solution, cache and SEO is very important for me.