ProtocolLib leads to players not being able to join

[tn-lorenz]

• [ ] This issue is not solved in a development build
• I'm using the lastest stable build

Describe the bug This only happens when trying to use ProtocolLib on a live-server (gportal), not on my local server. When ProtocolLib is installed, players trying to connect will face the "Logging in..." screen for about 2 Minutes, then will be greeted with "Timed out.". In the server console, only the "[16:43:20 INFO]: UUID of player [player] is [UUID]" message is printed, no join message and so forth, for example:

[16:37:09 INFO]: UUID of player is <UUID> [16:37:09 INFO]: [+] [player] [16:37:09 INFO]: [player][/[IP-Adress]:[Port]] logged in with entity id 98651 at ([Map]11.5, 118.0, -138.5)

There are no plugins installed, that mingle with packets. I have verified that the issue is caused by ProtocolLib by running it alone.

To Reproduce Steps to reproduce the behavior:

1. Install the plugin on a gportal Minecraft Server running Spigot 1.8.8
2. Try to join

Expected behavior Players should be able to join like normal.

Version Info Provide your ProtocolLib install info with /protocol dump through pastebin.

Additional context

• An error message that actually never was printed in the console before, but showed up, when trying to reproduce: https://pastebin.com/xb1y5Std (Actually I think this was probably caused due to accidentally having installed to versions of worldguard, however I have since removed the redundant jar and it still doesn't work)
• Sadly somehow the console doesn't really register what I'm typing (although rcon is enabled), so I cannot provide the contents of protocol dump.
• This is the latest log: https://pastebin.com/MfpBGPnW
• The issue is not present on ProtocolLib version 3.6.5