I've had this issue a few times with other encounters, but finally managed to reproduce it consistently. I have an encounter that has two homebrew monsters. I click on one and can see its description and stats. I click on the second monster and the contents of the page (everything between the menu header and the footer) disappears and is replaced by a "Report feedback" button. On the console, I get the following exception:

DOMException: Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.
    at Ss (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7012717)
    at Dc (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7024507)
    at t.unstable_runWithPriority (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7089159)
    at Zi (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6967597)
    at wc (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7022465)
    at yc (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7018551)
    at https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6967820
    at t.unstable_runWithPriority (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7089159)
    at Zi (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6967597)
    at qi (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6967765)
    at Yi (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6967700)
    at Ne (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:7038867)
    at Kt (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6946318)
    at HTMLDivElement.r (https://media.dndbeyond.com/encounter-builder/static/js/main.ddad94d8.js:2:6520101)

 I've replicated this on chrome and firefox. I can provide images if the explanation and provided example are not enough.

Also, after additional experimentation, I found that the issue does not even require two homebrew monsters. In this encounter, viewing the stats of a homebrew monster (Damia) on the encounter screen then trying to see the stats of any of the other two monsters also causes the issue. Visualizing the stats of the two other non-homebrew monsters in any order is fine and doesn't cause the crash.

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Edit: I am no longer sure this is an issue with homebrews in general. I've just created another encounter with a different set of homebrews monsters and the issue doesn't happen with them. With the encounters I've shared above, however, the problem happens consistently.

Please post links to any homebrewed monsters that cause the issue so a mod or staffer (not me) can look into them for you. They can even view unpublished homebrews so DON’T PUBLISH THEM.

I have shared links to the encounters themselves, which I believed were enough. If they are not, they are as follows:

Fist encounter, both monsters are homebrew: https://www.dndbeyond.com/encounters/4d89d02e-d88b-4ead-b0a9-c41d84b50266

(1x) https://www.dndbeyond.com/monsters/1160219-chaos-elemental

(2x) https://www.dndbeyond.com/monsters/555007-blink-troll

Second Encounter, only the first monster is homebrew: https://www.dndbeyond.com/encounters/90520417-8730-4dc1-83ca-a03938979b2e

(1x) https://www.dndbeyond.com/monsters/1153744-damia-agneia

(1x) https://www.dndbeyond.com/monsters/17339-necromancer

(1x) https://www.dndbeyond.com/monsters/16790-assassin

I can see both the Chaos Elemental and the Blink Troll, as well as Damia Agneia all normally when viewing your encounters. Are you still having the issue? If so, are you running any ad/popup blockers, browser extensions, or a VPN? Any of those might be interfering with DDB.

Huh, it appears that it only happens when I have the Beyond 20 extension activated. It looks like an issue with the extension, so I'll head over to their github to see if they manage to figure it out.

Thank you for the help!

Just in case anyone encounters this same issue, there is already an issue on the extension's github about it: https://github.com/kakaroto/Beyond20/issues/809#issuecomment-854390719