There was an error decrypting an OMEMO message addressed to this device. See the debug log for details.

[ZenithElevate]

In my infrastructure, there is a bunch of Pidgin clients on Windows, a bunch of Android phones with Conversation, and my one and only Linux with Pidgin. Some time ago, I migrated all of them from OTR to OMEMO, and everything worked well until a couple days ago when one and only one Windows Pidgin began to fail to deliver messages to my Linux Pidgin.
I can send to it fine. When it sends to me I get:

There was an error decrypting an OMEMO message addressed to this device. See the debug log for details.

This is what corresponds to that error in debug log:

(18:17:45) xmlnode: XML parser error for xmlnode 0x55a5fec21c90: Domain 3, code 100, level 1: xmlns: URI eu.siacs.conversations.axolotl is not absolute
(18:17:45) jabber: Sending (ssl) (~~~~~~@domain.com/PidginLinuxPC): 
<message type='chat' id='purple3bd8fc4e' to='^^^^^@domain.com'>
<active xmlns='http://jabber.org/protocol/chatstates'/>
<encrypted xmlns='eu.siacs.conversations.axolotl'>
<header sid='116#####42'>
<key rid='74#####12'>Mw......sQXg1.......2Dm/rR.....2Nm/R.....d.........a........g.......Q.zedA5ji7/qC.R............................................................................................................HsMFMG/SIcP3....................................EMAA=</key>
<key rid='12######12'>MwhKE....................................................................A5ji7/qCm...........................................................................................hqorP/3wnc.............................XIN/Lvs....................EMAA=</key>
<key rid='627####00'>M....e/w...T/ZKi.........................................................................................bLfQ/1yv...............6h4=</key>
<key rid='160#####48'>MwgdEiEF............................................IQUMJ5x4gf6JUEWIQ9zedA5ji7/qCmR.............................................................................1kE/Ih...n/1F.................................................................QwAA==</key>
<key rid='185#####79'>MwohBT................................................................TO8l/jZj.........Zs/6j/Dq........N//Go...................gGfk=</key>
<iv>5fl..........WOQ</iv>
</header>
<payload>z5FDaw==</payload>
</encrypted>
<encryption xmlns='urn:xmpp:eme:0' namespace='eu.siacs.conversations.axolotl' name='OMEMO'/>
<store xmlns='urn:xmpp:hints'/>
</message>

Pidgin folks don't know what Lurch does, so the question is to you how is it possible that only one way communication suddenly broke from only one client, and how to restore service?

[ZenithElevate]

Another system now has the same problem.

[gkdr]

Hi @ZenithElevate, thanks for the report.
This plugin has some dependencies, e.g. it uses libgcrypt for the crypto operations. Do you know if that was updated on any of the machines when it happened?

Regarding the output from the log: Did you put the dots in the keys? If not, this is invalid base64 encoding.
And that seems to be the output on the sending side, could you also paste the log from the receiving side when the error happens?

[ZenithElevate]

No, those libraries have been copied onto each of the machines, from the same location, and never touched since.
Those dots are mine.
The log is from the receiving side.