‘Unable to process request. Concurrent requests limit exceeded.’
This system exception will be raised when your concurrent Apex requests exceed governor limitations for your Salesforce organization. These limits are determined by your current Salesforce edition.
If Synchronous Apex runs more than 5 sec it considered as long running job. And we have limit that only 10 long running job can execute at a time. So, whenever 11th Synchronous apex tries to execute, it gets Concurrent Apex limit error
Possible concurrent Apex error scenarios:
Poorly coded Apex
Synchronous callouts from Visualforce pages
Inefficient SOQL queries
Check the below link for tips to fix the Concurrent Apex