You are right it does look more inclined to be client end. This can be a number of things, corrupt CVars or macro/s. Despite addons are disabled, they can still be responsible for a CVar corruption or error.
    Can you please install or enable BugSack and BugGrabber, with these enabled along with DugiGuides could also shed some light as to what’s going on. I am intrigued to see if there’s a lua error or taint lurking in the background.
    If you hear “Fatality”, click on the red sack minimap icon and then click anywhere in the middle of the output and press CTRL + A and then CTRL + C. On the desktop, right click a blank space and select New, and then select Text Document. Name it Crash. Open it and then press CTRL + V, click the X top right and then click save. Repeat this for other errors, of course, name the next error, Crash 2 etc.
    If you did get errors or taints, please click the Attach File in the
    WYSIWYG tool bar and then browse to the file and click add, repeat for
    the other files.

    I would like to try getting you to try typing /dugi fix followed by hitting enter and then click Okay on the reload prompt.
    Once you enter game after the reload, try mounting on a Dragonriding Mount, try not to use a macro and see if it happens again. If the problem persists, please try Reinstalling the Addon.

    Reinstall DugiGuides Addon:-

    1. Open Dugi Installer.
    2. Click on Settings button.
    3. Click on Reset button.
    4. Click Okay button x3 times.
    5. Click on Okay button.
    6. Click Reinstall button.
    7. Exit the Dugi Installer.

    I am looking forward to hear back from you with any results.