@DbLookup formula in dialog list: Server error: Entry not found in index

As usual, I am trying to populate my Dialog List field by using @-formula in it:

server:="WPRServer/Un";
dbPath:="Region/Users.nsf";
viewName:="Search_users";
@DbLookup("":"NoCache"; server:dbPath; viewName; "myKey"; 2)

But every time I am getting the error in my field:

Server error: Entry not found in index

I have tryed to use this formula to this field with another database on other server with different key names, but I am getting this error on this field again.

1. I have updated the views with CTRL + SHIFT + F9.
2. Tryed to update the current db design.
3. Recompiled LS and all views.

This is very strange, because I am using the similar formulas in other databases and everything is working fine every time.

Can you please give some small advise what is the way to fix this issue or maybe I am doing something incorrect? Thank you.

Update 17.09.2018:

1. The view Search_users is sorted.

2. I am using Windows server, and tryed to add **\** slashes and It helped to solve this issue, now I am getting this error:

 This database is currently in use by another person or process, and cannot be accessed at this time.  In order to share a Notes database, it must be accessed via a Domino Server by all users of the database.

I have tryed to reset all current accesses to this database with Domino Administrator tool, nothing helped - still getting this error in my Dialog List.

3. @DbColumn formula works fine with this view.

4. Also, already tryed to compact the database, no changes.

This may show you what the issue is:

server:="WPRServer/Un"; 
dbPath:="Region/Users.nsf"; 
viewName:="Search_users";
searchkey := "myKey";
rslt := @DbLookup("":"NoCache"; server:dbPath; viewName; searchkey; 2);
@If(@IsError(rslt); @Text(rslt) + " for:[" + searchkey + "]"; rslt);

Anticipation and handling error conditions.