Big problem with Model file: many message flow lines change after saving/re-opening file

Dear staff, We have a big problem...

Due to different circumstances we did not work on our Bizagi models for some years. However there is a need again..but we face huge problems.

1) The original Bizagi model file (it contains 40 diagrams) cannot be saved; It continuously gives the error "Error saving model: index was outside the bounds of the array". Impossible to solve for us. To save can only be done via "Save as", but that is not handy and feels strange. The error(s) still exists.When validating though, there is no error...

2) I created a new Model file, using version 4.0.0.065, and copy/ pasted each model into the new one, and in case of errors, fixed them. However we lost all attachments included in the models, all sub-process references, etc. and as the final disaster ... as soon as we save the model, and re-open it, many message flow connectors have changed positions and most models have become a mess! What has happened to Bizagi Modeler????

I really do not know how to solve all this... and I do not hope that we have just thrown away hundreds of manhours due to an unreliable software....

Please help, and how to solve/proceed?

Let me know if your need a copy of the file.

best regards, Carel