Cannot execute Test Suite Collection in parallel

All of my tests runn in all browsers, the suites run as well. But when I try to execute them in parallel from a Test Suite Collection, I get the following message

Test Cases/Execute search and view detail FAILED.
Reason:
com.kms.katalon.core.exception.StepFailedException: Unable to open browser with url: ‘’
at com.kms.katalon.core.keyword.internal.KeywordMain.stepFailed(KeywordMain.groovy:48)
at com.kms.katalon.core.webui.keyword.internal.WebUIKeywordMain.stepFailed(WebUIKeywordMain.groovy:65)
at com.kms.katalon.core.webui.keyword.internal.WebUIKeywordMain.runKeyword(WebUIKeywordMain.groovy:27)
at com.kms.katalon.core.webui.keyword.builtin.OpenBrowserKeyword.openBrowser(OpenBrowserKeyword.groovy:81)

and it continues…