Webmail - can login, but contents of Inbox inaccessible and not listed

[#1]

I've been a Plusnet webmail user for years.  I have run into a problem.

I can still log in normally.  I can see the menu list on the left showing Inbox, Sent, Deleted, and Junk, and the dozen or so sub-folders into which I have saved and compacted emails by subject which I wish to keep.

I can click on to, and access, all the emails in these sub-folders.

But when I click on the main Inbox  - which I know has a large number of as-yet unsorted messages - I cannot see the contents.  The little icon shows "loading", but the main display where the message-list should appear is blank.  I have tried the Search option, to pull up a selection of messages in order to delete them;  but the Search function doesn't work either.  On some occasions when I try to Search, a pop-up appears saying "Server Error".

I may possibly be getting close to the storage limit.  But I have had no alert or warning.  I have pruned the sub-folders, which I can access.  But now, since I cannot see or access the messages in my Inbox, I cannot delete any of them!  Catch 22.

How can I resolve this problem?

[#2]

Update.  

While waiting for advice I browsed the forum..... and found the post by Townman which is stickied on this forum.  It refers to slow loading problems with the Standard Email (RoundCube) system and says the fault is still being investigated.

I tried the actions recommended in that post - and hooray!  It solved, or at least improved, one of the problems.  The contents of my Inbox are now showing up in a list.

I am now trying to run Search (in order to bulk-delete messages).  That still doesn't seem to be working properly.