After working on a model for some time, suddenly the model cannot be opened and acts as if it is saved in a newer version:
"The model you are trying to open was saved in a newer version and cannot be opened in this version."
The problem is resolved by reinstalling the product.
This has been known to occur when using software deployment automation, which may be automatically reinstalling an older version of the product. For example if you are part of a "collection" for an older version of the product in Microsoft SCCM (System Center Configuration Manager), it may be silently trying to reinstall that old version. The error in question would then occur when opening existing models, because the models were indeed saved in a newer version and Bentley's water products are currently not Forward compatible. You can check for this by looking at the version number under File > Help > About (or Help > About for older versions). The version that your model was saved in can be found by opening the .WTG/.STSW file in a text editor. See: How to find the version a model was created in and last saved in
In this case, the user in question needs to be removed from the SCCM collection for the older version, then the newer version needs to be reinstalled. This would be done by your system administrator, typically in your IT/HelpDesk.
This could also occur if multiple users are working on the model and some users are using a newer version (though reinstalling would not help in this case, unless you also upgraded). For example if the model is stored in ProjectWise or a shared network folder. Check the version that the model was last saved in and compare to your own.