Thanks for the quick reply Shannon! We're on 1.2.0, does 1.2.1 contain important fixes regarding this?
We're having the same issue as described here: https://github.com/umbraco/Umbraco-CMS/issues/8766
And we see the exceptions from this TS. Must be related to the custom index we have, but what is wrong with it 😬

Well I've been switching back and forth between a custom index and adding fields to the existing umbraco members index.
With the default members index I was running into this issue: https://our.umbraco.com/forum/using-umbraco-and-getting-started/105368-customize-examine-index-not-being-used-on-boot
So that's why I went back to a custom index. It's also much faster to build than the default members index, I'm not sure why though.

I'm not rebuilding on startup or similar, just a custom index. This is the complete code:

using System;
using System.Collections.Generic;
using System.Linq;
using Examine;
using Examine.LuceneEngine;
using Examine.Providers;
using Umbraco.Core;
using Umbraco.Core.Composing;
using Umbraco.Core.Logging;
using Umbraco.Examine;
using Umbraco.Web.Search;

namespace ClientName.Core.Components
{
    public class ClientNameMembersIndexComposer : ComponentComposer<ClientNameMembersIndexComponent>, IUserComposer
    {
        public override void Compose(Composition composition)
        {
            composition.RegisterUnique<MemberIndexCreator>();

            base.Compose(composition);
        }
    }

    public class ClientNameMembersIndexComponent : IComponent
    {
        private readonly IExamineManager _examineManager;
        private readonly MemberIndexCreator _memberIndexCreator;
        private readonly ILogger _logger;
        private BaseIndexProvider _indexProvider;

        public ClientNameMembersIndexComponent(IExamineManager examineManager, MemberIndexCreator memberIndexCreator, ILogger logger)
        {
            _examineManager = examineManager;
            _memberIndexCreator = memberIndexCreator;
            _logger = logger;
        }

        public void Initialize()
        {
            foreach (var newIndex in _memberIndexCreator.Create())
            {
                _examineManager.AddIndex(newIndex);
            }

            const string clientNameMembersIndexName = Models.Constants.ClientNameMembersIndexName;
            if (!_examineManager.TryGetIndex(clientNameMembersIndexName, out var index))
            {
                _logger.Error<ClientNameMembersIndexComponent>($"Index {clientNameMembersIndexName} not found");
                return;
            }

            //we need to cast because BaseIndexProvider contains the TransformingIndexValues event
            if (!(index is BaseIndexProvider indexProvider))
                throw new InvalidOperationException("Could not cast");

            _indexProvider = indexProvider;

            _indexProvider.TransformingIndexValues += IndexProviderOnTransformingIndexValues;
        }

        public void Terminate()
        {
            if (_indexProvider != null)
            {
                _indexProvider.TransformingIndexValues -= IndexProviderOnTransformingIndexValues;
            }
        }

        private static void IndexProviderOnTransformingIndexValues(object sender, IndexingItemEventArgs e)
        {
            if (e.ValueSet.Category != IndexTypes.Member || !e.ValueSet.Values.ContainsKey("zipcode")) return;

            var zipcodeField = e.ValueSet.Values.Single(x => x.Key == "zipcode");
            e.ValueSet.Set("zipcode", zipcodeField.Value.Single().ToString().Replace(" ", string.Empty));
        }
    }

    public class MemberIndexCreator : LuceneIndexCreator, IUmbracoIndexesCreator
    {
        private readonly IProfilingLogger _profilingLogger;

        public MemberIndexCreator(IProfilingLogger profilingLogger)
        {
            _profilingLogger = profilingLogger;
        }

        public override IEnumerable<IIndex> Create()
        {
            var fields = new List<string>
            {
                "nodeName",
                UmbracoExamineIndex.NodeKeyFieldName,
                "email",
                "city",
                "zipcode",
                "houseNumber",
                "houseNumberAddition",
                "residenceType"
            };

            var index = new UmbracoMemberIndex(Models.Constants.ClientNameMembersIndexName,
                new UmbracoFieldDefinitionCollection(),
                CreateFileSystemLuceneDirectory("ClientNameMembers"),
                new CultureInvariantWhitespaceAnalyzer(),
                _profilingLogger,
                new MemberValueSetValidator(new[] { "Member" }, null, fields, null));

            index.FieldDefinitionCollection.TryAdd(new FieldDefinition("houseNumber", FieldDefinitionTypes.Integer));

            return new[] { index };
        }
    }
}