This warning is shown in Excellentable version 4.1.7 and lower and for the following scenarios :
- Importing a file with large number of cells
- Opening an Excellentable with large number of cells.
The permissible limit for Collaborative Editing to work is that Excellentable should have less than 300K cells, ideally without any formatting. If the number of cells is more than 300K, this warning is shown and Excellentable is switched into single user mode.