license error with full license file

[surveyer]

we deployed the Checkbox Server application (v4.4.1.14) on our server and replaced the trial license file with our full license file.
The red message indicating that the current installation is a trial license disappeared.
The problem ist that sometimes the following error message appears when a survey is started:
"The license for this installation could not be validated

No license file was found for this installation of Checkbox Web. Please check that there is a license file
(.lic file). in the /bin directory of this installation of Checkbox Web. You may download a license file for
Checkbox Web from our custom support area."
We could not find any regularity in the occurrence of this message. Sometimes it could be solved by closing the browser window and starting the survey again.
Is there any known workaround to prevent this issue?