If you are of sufficient channel or fleet rank, you can try the following commands (and steps) to restore your access to viewing and writing in channels:
(Note: Sufficient rank for a fleet member can vary because of different permissions, but to the best of my knowledge, for a custom channel, it is 'Admin' or 'Owner' -- 'Operator' does not count).
1) This one is for custom private/public channels -- Type: /channel_motd <channel name - no brackets> <message here - no brackets>
2) This one is for fleet chat -- Type: /Guild_SetMotd <message here - no brackets>
* Note: The commands are not case sensitive; the uppercase letters are just for ease of reading =)
After you do that, for custom channels anyhow, sometime you have to open your chat options and then the 'channels' tab (with or without even needing the 'refresh' button that is therein).
I've heard 98-100% success rate for the custom channels (although, the error can popup again later on - not sure why). However, the good news is that the same command can fix it, again, until it's fixed properly