MATLAB is unable to load MAT-file created by Dymola

It should not be a problem with the Matlab4 format. The number of rows and columns of a matrix are stored as 2 32-bit unsigned integers, which should make a double matrix of size 8*(2^32-1) *(2^32-1) possible. It is just an implementation issue for reading the format.

Given the number of data points, if the model has more than roughly 136 variables, the total size of the data_2 matrix is greater than 4GB (you can probably tell from the file size of GridHH_SLP.mat if this is true). If this is the case, maybe Matlab was not programmed to handle matrices of this size. I tried a 39GB data_2 matrix in Octave and it just says: error: out of memory or dimension too large for Octave's index type. I could open the same file in OpenModelica (but it is inefficient for plotting such large files because they are not saved in transposed format by default).

If Dymola is capable of outputting a fewer number of variables, the file could work fine in Matlab. You could also try the program alist that comes with Dymola to output select variables in a mat-file into csv format (if alist can handle large files).