Same issue here on latest Mojave with a 13" touchbar Macbook Pro. 5.1.1 introduced more issues than it fixed.

same issue
 

I have the same problem, but I don't see how I can fix it.

Help please!

PLANNED · Wim (Product Manager, DisplayLink) responded
 

https://support.displaylink.com/foru...re-suggestions

Fix "WindowServer [ERROR] - Unable to find display pipe for IOFB service: 18715" on MacOS
DisplayLink do not work properly with 4K screens @ MacOS 10.14. It looks laggy, jamming, like FPS drop.
I've described issue in details here: https://displaylink.org/forum/showthread.php?t=66349

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PLANNED · Wim (Product Manager, DisplayLink) responded
We’re aware of this error message and have investigated if there is any underlying problem or performance impact. We don’t believe that there any actual problem here, and this message is not responsible for performance issues reported in this thread.

However we can’t find a way to easily suppress the message. We know this is annoying and would like to stop it appearing so we’re continuing to look at ways to stop it.