Code Coverage issues after merging .db and generating reports

Hello, I must first express my gratitude for the software and the support provided; it's been fantastic. I've been simulating an RTL design with an UVM testbench this lasts weeks, and I've recently activated code coverage. I followed the advice in this post to focus solely on the RTL: code coverage causes to crash : Support (metrics.ca). However, I'm encountering an error when attempting to merge three .db and then generate reports. Could you assist me in understanding what might be causing these issues? 

The design remains consistent, and the parameters have not been altered. This is the output after running dc_report command, html reports seems to be generated partially.

Also I noticed that sometimes index.html file is empty and the rest of the specific html reports of that design are present. I'm not sure if its a known issue or not.

Thanks in advance,

Ricardo