Error 500 could not obtain file lock in class_datastore.php - File cache is active

[postcd]

Hello,

please what would You try if you see random 500 Internal Server Error

Transferred275 B (0 B size)
Referrer Policystrict-origin-when-cross-origin
x-powered-by: PHP/5.6.40
content-length: 0
Cache-Control no-cache
Pragma no-cache

for these elements of the vBulletin 4.2.5 admin control panel:
admincp/index.php?do=home
admincp/index.php?do=nav
admincp/index.php?do=head

and the error log showing: could not obtain file lock in class_datastore.php at line highlighted below

if ($this->lock())
{
...
}
else {
trigger_error('Could not obtain file lock', E_USER_ERROR);
}

Should i ask my hosting provider about any particular PHP/Apache variable detail or some vBulletin admin action (i have tried to edit and disable enable plugin product and clear cache already). Also if it can be caused by file/folder permission, which one to check please?

includes/config.php has commented out all lines in sections:

// ****** DATASTORE CACHE CONFIGURATION *****
// ******** DATASTORE PREFIX ******

except: $config['Datastore']['class'] = 'vB_Datastore_Filecache';

includes/datastore/datastore_cache.php is 644 (tried to chmod it 777 without luck) and parent folder 755. i have not checked immutable bit yet.
On other site admincp shows: Fatal error: Could not obtain file lock in [path]/includes/class_datastore.php , but this error seems to be random! appears, then ok a few refreshes.

when i comment out that line "$config['Datastore']['class']" by prefixing it //, then the admincp start loading faster and without error 500. So how to still cache?

http://dev.vbulletin.com/docs/html/main/datastore
datastore folder, file permissions are 755, 644

Thank You in advance for help