Dear @dharr, indeed, this could be an explanation. Thanks a lot!

Dear @janhardo, the most concerning part for me is that, as documented in the old documentation, the system of PDEs possesses a four-dimensional solution space. According to the most recent version of the package (as distributed with DETools), there is only a trivial solution... From a mathematical point of view, it is very disturbing.

Dear @janhardo, if the old package can be downloaded from somewhere, I can see only this URL address:

https://wayback.cecm.sfu.ca/~wittkopf/standardform.html

By the way, I emailed the authors and never heard back from them...

Best regards,

Denys D.

@dharr, Thank you very much for your reply. It clarifies a lot the situation. What a relief!