That is a very strange issue since the file the application plays through the browser is the same file you download. I have never seen/heard of this error before. I wonder if you have anti-virus software that is scanning downloaded files and perhaps it is somehow interfering with the download process? That's a total guess since I have never seen this problem in the nearly 10 years that we have been offering Recording Manager.
What version of 3CX are you running V16? Windows or Linux?
In 3CX 15.5 we exposed the 3CX recordings to Recording Manager through a network shared folder. In version 16 we download the recordings through the Relay so a shared folder is no longer needed (that's a good thing). Since the process has changed significantly in V16, we will need to look at this closely. Please contact the support department to schedule some time for us to look at your environment: Support@VoIPTools.com or use our Live Chat feature during regular business hours.
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