I cannot access ACP after the update to 4.0.13.1  and I cannot change themes either as the dropdowns do not work at all

Nasty update

If you upgraded from 3.x, make sure you're using your Display Name or Username, whichever you upgraded. 

If coming from a previous IPS4 version, are you using a custom theme? 

I think you have several options.
Ticket Client Area
Link your forum - if someone get an idea ...
What error logging into the ACP?

I ended up restoring a backup of the forum for now

I will wait till 4.1 is ready, then I will upgrade and take it from there

It is possible to upgrade package you received with an error.
Maybe you should download again and try 4.0.13.1 upgrade.

It is possible to upgrade package you received with an error.
Maybe you should download again and try 4.0.13.1 upgrade.

I just to let you know I appreciate your advise

I will try as you suggested some time when its not so busy and report back here on the results

I had the same problem and deleted the cache manually and everything went back to working.

Edited September 12, 20159 yr by dmaidon

I cannot change themes either as the dropdowns do not work at all

Nasty update

I did fill bug report about that but as yet still pending

It is possible to upgrade package you received with an error.
Maybe you should download again and try 4.0.13.1 upgrade.

I had more issues so I used the support function in ACP and went through the steps and eventually downloaded and installed the update again, which now works

I did the update as I had issues with commerce and setting default theme, which didnt rectify itself at the upgrade, so I have contacted client support.

I use a theme from IPBforo and I have had issues with it, reported it to the dev...

The problem is... as I cannot change the default theme to thr original IPB default theme, means I cannot delete the IPBforo theme....

Arrrrghhhhhh

Try this:
Copy theme IPBforo -> IPBforo2

Make IPBforo2 your default theme.

 It is possible your issues disappear.