rfm_ArrayRepr Error

Hello, 

I get the following message when extracting results via a python file : 

'rfm_ArrayRepr : : rfm_ArrayRepr : Handle is not to Array object in ODB/CAE
file' 

This message follows an explicit calculation under Abaqus 2018 with 2 steps
and 5000 frames per step. 
The calculation is carried out without apparent error and all results are
available under the CAE. 
When extracting the results in displacement and effort, everything rightly
runs except for one frame. 
For example, if we consider frame 1500 in step 2, we get the above message. 
However, the results are available in frame 1499 and 1501 and the following
ones. 
I have the same problem for other files but at different frames. 
Has anyone ever encountered this problem? 

Regards, 
Philippe