A couple of weeks ago we noticed issues with multiple reports using live connection to a particular SSAS Tabular model. Some measures in them would start returning an Unexpected Exception, or sometimes spin until the server ran out of memory, but other times were fine. We went crazy trying to figure out if the issue was in the reports, model, gateway, or SSAS server configuration. Eventually we determined the following was happining:
A new report using a live connection to the model had been built, and since we needed a custom measure for this particular department, we had used the new report level measures. It turns out that refreshing this particular report with the new measure in it would cause some other measures in the model to return "Unexpected Exception" for all future queries, whether they were from Power BI reports, Excel workbooks, or SSMS queries. Reprocessing the model would correct the issue, until the next time that particular report was refreshed.
We removed all report level measures from the report and no longer see the issue, but obviously are not going to use report level measures again until this issue is resolved.