Yes
I mean convert epub to epub. It helps me lot of times also when it was original epub from publisher. I tried lot of programs for converting but Calibre was always better.
Here is exact instruction from Pocektbook help
10/2019)
"In this case we recommend to perform internal memory formatting.
Settings - Maintenance - Format Internal Memory.
Please note, when formatting all user files will be deleted.
To save user files you need to copy them on your computer or Micro SD card.
Connect to computer, select PC connect, open Pocketbook folder and copy all user files.
Or open library on your device, select folders, press the necessary file for about 3 seconds and choose File - Move - Back - SD card - Select."