Searched the web a little further. Refs not supported on san-storage? 
Not yet recommeded for clustered volumes due "redirected mode" ( Redirected mode: can't bypass...

Refs is supported but I can't find more information. Have you tried it in a test-environment?
Refs seems to have a memory impact

SQL will likely be faster as PL/SQL causes cpu-context-switches.
Perhaps they made cursors for the error-handling?

Disabling reportlogin and killing reportsessions sounds feasible.
Another option is read committed snapshot isolation ( depends on what kind of ETL is running)

Can't really recommend further steps, as you checked a lot.
Anything odd on the VM(host) side? Like not running in high-performance mode, overcommitment, old drivers ...
Maybe ssrs is...

Hello,
If you can't optimize the reports Troubleshooting Reports: Report Performance
you might want to check SSRS subscriptions under the hood
It mentions MaxQueueThreads in the...

I don't have a compare case Hyper-V versus VMWare
You might want to check the setup of the Hyper-V host , the Hyper-VM , cpu, network, attached storage
MS realises...

Export to csv to minimize rendering.
Preaggregating on the database to limit size?

Chunkdata=temporary data fetched for report like for showing page Y of Z. If you can'r restore ,  repair_allow_data_loss will probably clean out (some of) the Chunkdata table as the clustered index is...

Curious what the SSRS native portal will look like since SharePoint integrated mode got dropped. (version management, tags replacements?)

For backdated changes we have an additional system that performs statistical analysis on both sides to detect missed items (count by created date, count by updatedate, count by status ...)....

If you want the latest information:
Full backup production old -> new (can be done beforhand) with norecovery
Differential backup old ->new with no recovery
Taillog backup old ->new with...

Congratulations Koen

I don't have the details as the infrastructure team was assigned to it. After excluding other sources, the plausible cause was software.

The antivirus vendor supplied a diagnostic utility (to detect...

It was Norton antivirus some months ago. Can't tell the version

They have released a new version in november 2016, which didn't have the problem